Opt-in per-process spectre mitigation is already the case for some of them, because the mitigations in question are way too costly.

Now it is not possible for every kind of mitigations, because e.g. patching the kernel between mitigated processes and unmitigated ones would be more costly than just always running the mitigations.

edit: thinking more about it: you could have crazy ideas like two versions of the whole kernel space always loaded :D not sure about the cache impact in this case though.