Looking at a few old mailinglists agrees with you. Apparently the mga_drv and mga_hal_drv could just be copied to any system running XFree86, at least Linux and

Looking at a few old mailinglists agrees with you. Apparently the mga_drv and mga_hal_drv could just be copied to any system running XFree86, at least Linux and the BSDs, and would let you utilize dual-head support.