*If* this was added to the PEP, I would add it as a new optional
".localN" suffix, with a recommendation that public index servers MUST
disallow use of local numbering, since it is intended for downstream
integrators to indicate the inclusion of additional changes relative
to the upstream version. Outside a given integrators environment, the
local numbering is no longer valid.

Seems like you have to add something like this, if you're not wanting "provides" to cover it.
Like you mention, ".post" won't work in all cases.
Otherwise people are left not changing the version, or changing the version in a way that could break consistency.