Short Question:
Is it possible to run Houdini Apprentice on a FreeBSD system?

Unfortunately no. Unless there is a huge demand for a port, no.

I would imagine the OpenGL drivers from the graphics card vendors would pose your biggest problem. Too much momentum with Linux right now.

What about X11 for Mac OS X?

Could Apprentice 6 work with X11 for Mac OS X downloaded?

Thanks.

No, we don't support the Mac OS even with X11. Binary incompatible.