In fact the 9900 itself was used in an entire line of minicomputers that included a hardware memory map.

The 9900 is a single chip implementation of the CPU board in the TI 990. They even created a dedicated memory mapper chip to go along with the product line, though it is significantly different than the one in the minicomputer line.

(edit: the 990 was first built in the early 70s, memory mappers are quite old conceptually)

(edit 2: in fact the necessity of using a memory mapper is what killed the platform, and was one of the things that made the IBM PC team decline the 9900.)

Unfortunately I don't think that there is a reasonable way to perform real hardware-level memory protection with that chip alone. I'm working on a project documenting the genesis of the 99000 chips, which include a privilege bit in the status register, from the minicomputer line.

Essay forthcoming, and probably an OS. Maybe a year...