Additional API from clementine:
https://github.com/clementine-player/Clementine/blob/master/3rdparty/qtsingleapplication/qtsingleapplication.patch

https://github.com/clementine-player/Clementine/issues/291#issuecomment-29984507
qtsingleapplication: "Open With" on the file browser, and passing filenames as arguments in the command line
Controlling playback with the commandline as well (--pause, --play, etc.)

--- QtSingleApplication-2.6.1/src/qtlocalpeer.cpp~	2015-02-23 00:06:20.000000000 +0200
+++ QtSingleApplication-2.6.1/src/qtlocalpeer.cpp	2015-02-23 00:08:51.179884397 +0200
@@ -42,6 +42,7 @@
 #include "qtlocalpeer.h"
 #include <QCoreApplication>
 #include <QTime>
+#include <QtDebug>
 
 #if defined(Q_OS_WIN)
 #include <QLibrary>
@@ -138,6 +137,11 @@
 
 bool QtLocalPeer::sendMessage(const QString &message, int timeout)
 {
+    return sendMessage(message.toUtf8(), timeout);
+}
+
+bool QtLocalPeer::sendMessage(const QByteArray &message, int timeout)
+{
     if (!isClient())
         return false;
 
@@ -160,9 +164,8 @@
     if (!connOk)
         return false;
 
-    QByteArray uMsg(message.toUtf8());
     QDataStream ds(&socket);
-    ds.writeBytes(uMsg.constData(), uMsg.size());
+    ds.writeBytes(message.constData(), message.size());
     bool res = socket.waitForBytesWritten(timeout);
     res &= socket.waitForReadyRead(timeout);   // wait for ack
     res &= (socket.read(qstrlen(ack)) == ack);
@@ -189,10 +190,10 @@
         delete socket;
         return;
     }
-    QString message(QString::fromUtf8(uMsg));
     socket->write(ack, qstrlen(ack));
     socket->waitForBytesWritten(1000);
     socket->waitForDisconnected(1000); // make sure client reads ack
     delete socket;
-    emit messageReceived(message); //### (might take a long time to return)
+    emit messageReceived(uMsg); //### (might take a long time to return)
+    emit messageReceived(QString::fromUtf8(uMsg));
 }
--- QtSingleApplication-2.6.1/src/qtlocalpeer.h~	2015-02-23 00:12:20.000000000 +0200
+++ QtSingleApplication-2.6.1/src/qtlocalpeer.h	2015-02-23 00:19:44.407242485 +0200
@@ -55,11 +55,13 @@
     QtLocalPeer(QObject *parent = 0, const QString &appId = QString());
     bool isClient();
     bool sendMessage(const QString &message, int timeout);
+    bool sendMessage(const QByteArray &message, int timeout);
     QString applicationId() const
         { return id; }
 
 Q_SIGNALS:
     void messageReceived(const QString &message);
+    void messageReceived(const QByteArray &message);
 
 protected Q_SLOTS:
     void receiveConnection();
--- qtsingleapplication-2.6_1-opensource/src/qtlockedfile_win.cpp	2009-12-16 10:43:33.000000000 +0000
+++ qtsingleapplication/qtlockedfile_win.cpp	2010-07-10 16:26:33.000000000 +0100
@@ -65,7 +65,7 @@
 
     Qt::HANDLE mutex;
     if (doCreate) {
-        QT_WA( { mutex = CreateMutexW(NULL, FALSE, (TCHAR*)mname.utf16()); },
+        QT_WA( { mutex = CreateMutexW(NULL, FALSE, (WCHAR*)mname.utf16()); },
                { mutex = CreateMutexA(NULL, FALSE, mname.toLocal8Bit().constData()); } );
         if (!mutex) {
             qErrnoWarning("QtLockedFile::lock(): CreateMutex failed");
@@ -73,7 +73,7 @@
         }
     }
     else {
-        QT_WA( { mutex = OpenMutexW(SYNCHRONIZE | MUTEX_MODIFY_STATE, FALSE, (TCHAR*)mname.utf16()); },
+        QT_WA( { mutex = OpenMutexW(SYNCHRONIZE | MUTEX_MODIFY_STATE, FALSE, (WCHAR*)mname.utf16()); },
                { mutex = OpenMutexA(SYNCHRONIZE | MUTEX_MODIFY_STATE, FALSE, mname.toLocal8Bit().constData()); } );
         if (!mutex) {
             if (GetLastError() != ERROR_FILE_NOT_FOUND)
--- qtsingleapplication-2.6_1-opensource/src/qtsingleapplication.cpp	2009-12-16 10:43:33.000000000 +0000
+++ qtsingleapplication/qtsingleapplication.cpp	2010-07-10 16:23:53.000000000 +0100
@@ -144,6 +144,7 @@
     actWin = 0;
     peer = new QtLocalPeer(this, appId);
     connect(peer, SIGNAL(messageReceived(const QString&)), SIGNAL(messageReceived(const QString&)));
+    connect(peer, SIGNAL(messageReceived(const QByteArray&)), SIGNAL(messageReceived(const QByteArray&)));
 }
 
 
@@ -265,6 +266,11 @@
     return peer->sendMessage(message, timeout);
 }
 
+bool QtSingleApplication::sendMessage(const QByteArray &message, int timeout)
+{
+    return peer->sendMessage(message, timeout);
+}
+
 
 /*!
     Returns the application identifier. Two processes with the same
@@ -291,10 +297,14 @@
 void QtSingleApplication::setActivationWindow(QWidget* aw, bool activateOnMessage)
 {
     actWin = aw;
-    if (activateOnMessage)
+    if (activateOnMessage) {
         connect(peer, SIGNAL(messageReceived(const QString&)), this, SLOT(activateWindow()));
-    else
+        connect(peer, SIGNAL(messageReceived(const QByteArray&)), this, SLOT(activateWindow()));
+    }
+    else {
         disconnect(peer, SIGNAL(messageReceived(const QString&)), this, SLOT(activateWindow()));
+        disconnect(peer, SIGNAL(messageReceived(const QByteArray&)), this, SLOT(activateWindow()));
+    }
 }
 
 
--- qtsingleapplication-2.6_1-opensource/src/qtsingleapplication.h	2009-12-16 10:43:33.000000000 +0000
+++ qtsingleapplication/qtsingleapplication.h	2010-07-10 16:23:53.000000000 +0100
@@ -91,11 +91,13 @@
 
 public Q_SLOTS:
     bool sendMessage(const QString &message, int timeout = 5000);
+    bool sendMessage(const QByteArray &message, int timeout = 5000);
     void activateWindow();
 
 
 Q_SIGNALS:
     void messageReceived(const QString &message);
+    void messageReceived(const QByteArray &message);
 
 
 private:
--- qtsingleapplication-2.6_1-opensource/src/qtsinglecoreapplication.cpp	2009-12-16 10:43:33.000000000 +0000
+++ qtsingleapplication/qtsinglecoreapplication.cpp	2010-07-10 16:32:33.000000000 +0100
@@ -81,6 +81,7 @@
 {
     peer = new QtLocalPeer(this);
     connect(peer, SIGNAL(messageReceived(const QString&)), SIGNAL(messageReceived(const QString&)));
+    connect(peer, SIGNAL(messageReceived(const QByteArray&)), SIGNAL(messageReceived(const QByteArray&)));
 }
 
 
@@ -94,6 +95,7 @@
 {
     peer = new QtLocalPeer(this, appId);
     connect(peer, SIGNAL(messageReceived(const QString&)), SIGNAL(messageReceived(const QString&)));
+    connect(peer, SIGNAL(messageReceived(const QByteArray&)), SIGNAL(messageReceived(const QByteArray&)));
 }
 
 
@@ -133,6 +135,11 @@
     return peer->sendMessage(message, timeout);
 }
 
+bool QtSingleCoreApplication::sendMessage(const QByteArray &message, int timeout)
+{
+    return peer->sendMessage(message, timeout);
+}
+
 
 /*!
     Returns the application identifier. Two processes with the same
--- qtsingleapplication-2.6_1-opensource/src/qtsinglecoreapplication.h	2009-12-16 10:43:33.000000000 +0000
+++ qtsingleapplication/qtsinglecoreapplication.h	2010-07-10 16:32:33.000000000 +0100
@@ -62,10 +62,12 @@
 
 public Q_SLOTS:
     bool sendMessage(const QString &message, int timeout = 5000);
+    bool sendMessage(const QByteArray &message, int timeout = 5000);
 
 
 Q_SIGNALS:
     void messageReceived(const QString &message);
+    void messageReceived(const QByteArray &message);
 
 
 private: