I think "somewhat incomplete" is underplaying it as such a large proportion of message exchange tends to include pointers to shared structures. That the message handling uses intrusive lists doesn't help. So "simple" really depends on where you set the bar for what you consider useful levels of memory protection.

I still think it's possible but doing it to a meaningful extent would be a long tedious effort, at least for my definition of meaningful.

You have Janus-UAE which sandboxes/emulates M68k apps for AROS "rootlessly" in that it bridges a number of structures etc. That mostly solves the problem on modern hardware for legacy apps, though it's a painful solution.

Then you could aggressively upgrade message formats for "native" apps to replace pointers with opaque handles.

If done at the time you could of course had relied on more of the developers of various apps still being active and willing to update their code and might have been able to avoid the Janus-UAE approach, but there was never a point at that time where anywhere close to a majority of users had MMU's,