Getting back to document conclusions of my research on best practice of using RaspiComm RS-485 under Qt for anybody, who finds this thread useful (I expect many of it is valid for RS-232 on same board as well, did not test myself):
Do not try to implement communication with MAX3140 UART chip yourself (maybe except of asm based implementation, what is however most probably unnecessary exertion). I succeeded to implement it based on kernel /dev/spidev0.0 only to find out that even this partially system driver supported solution is too slow for even 19 200 Bd speed (loosing about 5 characters on one received).
I myself feared about RaspiComm RS-485 driver /dev/ttyRPC0 based on some forums complaints and not clear versioning, however found out, that the forum thread was heavy outdated. These official installation instructions worked like a charm and the resulting driver worked out-of-box for my latest Raspbian Wheezy (4.1.7+ #817). So I strongly recommend to take this approach.
QSerialPort class has constructor with signature QSerialPort(const QString &, QObject *), where QString may contain even device which is not included in the QList that QSerialPortInfo::availablePorts() returns. This works with no surprise. If one does not set anything than (eventually) baud speed, one gets standard 8 bit, 1 stop bit, no parity without any character translations, good old plain raw binary. So QSerialPort may be used with /dev/ttyRPC0 directly and easily.
rpc0.write("Binary request\0even\xffcontaining weird\x03characters", length_of_binary);
if(rpc0.waitForReadyRead(100)) // Enough long time in miliseconds
QByteArray data = rpc0.readAll(); // You may have to wait/read repetitively in loop
// and merge data on higher link speeds
Hardware of RaspiComm uses RTS pin to select in/out direction of data. CTS is kept active permanently by wiring, so using default use RTS/CTS mode works properly with no inappropriate blocking.
That's all you should need, so enjoy your communication.