Even switching back and forth at the right times seems somewhat risky to me before ExitBootServices(), while Runtime Services are restricted to a subset that's

Even switching back and forth at the right times seems somewhat risky to me before ExitBootServices(), while Runtime Services are restricted to a subset that's much more resilient to external state changes (and doesn't run background stuff). NetBSD rump kernels may provide a better base for OS experiments where you can use working code until you have replacements ready - based on a real OS, not some API that accidentally became one.