As the Fossil Debian package maintainer, I'd certainly be grateful for notes on how to use Fossil for Debian packages. I have my own idiosyncratic workflow for Fossil itself, but it's not general because it relies on upstream's distribution of tarballs, and otherwise uses the local fossil repo, and is not very automated. Would be happy to put notes, scripts, snippets, etc, in /usr/share/doc/fossil/README-Debian or some such.
A pipeline with "v=$(fossil tag list | egrep <version-tag> | max) ; fossil tarball $v foo.tar" where <version-tag> defaults to version_.* or some such but can come from debian/gbp.conf might be useful, for use with a local fossil repo. If there's nothing local only remote, hmmm.... One thing I should mention is that Fossil upstream is very cooperative. If the web pages a Fossil repo makes available by default don't have the right hooks for uscan, if we could ask for something very simple (maybe a page with a list of all tags each consisting of a link to a tarball FOO-tag.tar.xz) that would I think be something they would seriously consider. --Barak.