`git describe` in an export-subst format string?

I originally posted this as a question about getting `git log` to
output the same thing that `git describe` does to the git-users
mailing list, and was directed here. The actual thing I want to do is
something like this:

  static const char *project_version = "$Format:...$";

where "..." is something that `git archive` will replace with the
output of `git describe`, given the file has the export-subst
property, possibly with arguments to git describe (--tags, --all,
etc.). In a normal build I can do this with the build system just
calling git directly, but if I'm archiving a tarball off there doesn't
seem to be a way to do this (other than manually replacing the format
string myself as some secondary step).

Does this seem like a reasonable thing to do, or am I overlooking a
better alternative? If this is something that seems reasonable, I
wouldn't mind taking a stab at implementing it. If that sounds okay,
could make sure I'm pointed in the right direction? E.g., should `git
log --format=...` have options that spit out `git describe` strings
since that's what export-subst currently uses, or should there be a
separate $Format:...$ option that runs git describe?

Thanks,
John