His MP ideas would only work if you'd heavily restrict the way allocated memory can be used. You'd need to have a flag that would indicate private memory. This memory could only be used locally. Even then it'd mean that each task would need to have a private memory pool to AllocMem from (since of course you cannot share allocations between tasks, thus it isn't as simple as "AllocMem() with memory protection active is supposed to be just AllocPooled() from system default pool.").
However, for many things to work the memory would need to be global, this includes Messages and any other structures that are being passed to the OS or are being included in some resource that might be accessible by other tasks.
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.