is there any reason a plugin couldn't • build a .pex • run the .pex to do codegen Sort of like an 'internal subsystem'

That is done here for mypy-protobuf codegen of protobuf type stubs: https://github.com/pantsbuild/pants/blob/7c1c9641fc216e5c4163dab09cb644fce287877f/[…]ts/backend/codegen/protobuf/python/python_protobuf_subsystem.py

yissss