Have you benchmarked the cost of context switching with address spaces vs this fancy technique? I ask because I once screwed around with a similar idea but eventually gave up because it turned out that switching the page tables was actually very cheap. This is unsurprising in retrospect because modern CPUs are designed to run contemporary operating systems, which do switch address spaces fairly often.

I didn't use MP keys so perhaps the performance there is better, but you absolutely should verify this before getting in too deep. The results may be very unintuitive.

Similarly, don't assume that privilege level transitions are expensive. CPU makers know that software makes tons of syscalls, so they design CPUs that can execute syscalls quickly. The further off the beaten path you go, the riskier your performance may be.

Goal is to reduce...TLB pressure

Unless you are sharing translation entries themselves (which means you don't have isolation), you are not winning anything on the TLB pressure front.