Alright, I might be completely on crack here (wouldn't be the first time):

One thing I've been pondering in regards various minimal embedded OSes like FreeRTOS, µcLinux, chibi-OS, Minix etc, is the possibility of running them *as a process* under Mac system software - much like the old MachTEN Unix.  They'd launch as a program, then sieze control of the CPU (unco-operative multitasking ), and make Mac Toolbox / System calls as required for display, IO, etc.