Such MP would be limited: It'd still be extremely trivial to take down the whole OS. As such I don't quite see the point of such MP.
The only way to get true MP is to totally scratch all of the AmigaOS API legacy.
You could attach a pool to a message port so that sender can allocate memory that is also accessible by receiver but not by other memory spaces. It is true that programs not using these features won't be protected and can bring down themselves and all non-protected programs. I do think it should be possible to have programs using the MP protected and not be brought down by other programs by implementing proper exception handling when they receive wrong data.
greets,
Staf.
