I think you could do in src/ongetpass.cpp:x2goMain

int x2goMain ( int argc, char *argv[] )
{
     QApplication app ( argc,argv );

before calling 'QApplication app(argc, argv);' do a setenv or something 
like:

int argcx = argc + 2;
char **argvx = new char*[argcx];
std::memcpy(argvx, argv, argc * sizeof(char *));
argvx[argc] = const_cast<char *>("-platform");
argvx[argc + 1] = const_cast<char *>("xcb");
QApplication app(argcx, argvx);

-- 
Orion Poplawski
Manager of NWRA Technical Systems          720-772-5637
NWRA, Boulder/CoRA Office             FAX: 303-415-9702
3380 Mitchell Lane                       orion@nwra.com
Boulder, CO 80301                 https://www.nwra.com/