I was studying how #tripos scheduler works this morning. I've never used a preemptive kernel that *didn't* support round-robin scheduling before (AmigaOS supports round robin scheduling).
Tripos is definitely the simpler of the two, by a *long* way, and as long as you are strictly I/O bound, every bit as powerful as with round-robin schedulers.
(Or, put another way, you can have *one* CPU-bound task as long as it's the lowest priority task in the system.)
You can certainly run Tripos with time-slicing and round-robin scheduling if you're willing to deal with the subtle complexities that arise (e.g., AmigaOS does exactly this).
Another work-around is to have a high-priority task that makes use of the timer to periodically call ChangePri() on a set of supervised (presumably CPU-bound) tasks to dynamically balance their priorities. This may be somewhat hacky, but it's not too far from microkernels with external schedulers, when you think about it.
Vertigo