Can't you just re-direct stdout or stderr? I'm sure that works even on Windows. E.g.

py -${PY_QUALIFIER} -m compileall -q "$INSTDIR\pkgs" 2> null

Is there a reason you cannot do this? I think that adding functionality to compileall to duplicate what your OS already provides should be a last resort.

I also don't understand what you mean by "it exits early". What exits early? Surely python doesn't exit until compileall finishes running. If it does, that's a bug.