JdeBPSounds a bit complex. (-:
Debian apt-ftparchive source actually makes checksum lists for source packages.
No handy equivalent in pkg repo, though.
It's not going to be high on the priority list. I could possibly modify the .do scripts to do something non-standard.
In the meantime, the HTTP Last-Modified: response header is right and the If-Modified-Since: request header is respected by Bernstein publicfile. For what that's worth.