Okay, thanks everyone who answered.
In hindsight you are all correct, writing a new PEP is the best solution
and I was being over-optimistic (and a little lazy) to think otherwise.
I think that, technically, I still have core dev permissions, even
though I haven't used them for quite some time. If nobody objects, I
would like to use them to sponsor the new PEP.
Does it make a difference if I am a co-author of the new PEP, or a
significant contributor?
Dipping-my-toes-back-into-CPython-dev-ly y'rs,
For sponsorship it makes no difference. A PEP needs to have either a sponsor or a co-author who is a core dev.
If you're *just* a sponsor you could still be appointed PEP delegate by the SC. But not if you're an author that can't happen. (Before we had an SC I occasionally approved my own PEPs. PEP 572 was the last such.)