Here I attach a new patch with what I think is the best approach to
handle the extra info for projects. This implementation is pretty simple
for both sides (user, project.el and backends) and uses your main idea
of the generics + a custom.
However I decided not to use only the custom as you suggested, because
considering the project initialization it will require a complex and
error prone approach to initalize the backend variables correctly (for
example, when some buffers are already open).
With this implementation the backends only need to define the optional
generic specializations and the policy is pretty simple to explain (see
the doc strings).
I also modifies the backend project on github to match this new api, so,
you can check it as well.
project-build-functions.diff