MPlayer runs on a number of exotic platforms, but most of the platform-specific changes from those systems have not been merged back into the main source tree. For build instructions you should refer to the system-specific documentation.
You'll need to download and install SDL for QNX. Then run MPlayer with -vo sdl:driver=photon and -ao sdl:nto options, it should be fast.
The -vo x11 output will be even slower than on Linux, since QNX has only X emulation which is very slow.
The people over at www.amigasoft.net make current MPlayer and MEncoder packages.
Nicholas Det at Genesi has done a big and powerful port of MPlayer for MorphOS. Sadly it's based on the 0.90 series.
Get if from MorphZone: