+1 for documenting the clinic DSL as a PEP. It's a bit too hard to find the rationale for toolchain changes like this when they're hidden away in tracker issues.

It doesn't need to be an extended essay like most of the PEPs I write, it can be relatively short and sweet (consider the PEP that approved the addition of unittest.mock: http://www.python.org/dev/peps/pep-0417/)

As Greg suggested, taking the current clinic.txt and publishing it as a "DSL for C API argument processing" PEP would get you most of the way there.