Just FYI, prefetch instructions are already a thing (at least on x86). At present they're only hints to the underlying heuristic system though. (https://www.felixcloutier.com/x86/prefetchh)

Cache-as-RAM is also a thing, and allows the sort of pinning you described. Pretty specialized use cases though. (https://stackoverflow.com/questions/41775371/what-use-is-the...)