I don't know any tooling that turns (recursively) a Maven pom file into a Debian repository of Debian policy-abiding .debs, which are magically updated when the pom file changes.

If you mean Debian policy-abiding in the sense of "signed by a Debian developer in the Debian WoT" then no, but that's not something you could ever do automatically. But for the rest, I've done all the individual pieces before: it is trivial to generate a .deb from a maven pom and put it in a debian repository, it's trivial to do some operation on all the dependencies of a maven project, and it's trivial to hook something into a maven repository to happen whenever a new artifact is uploaded. You absolutely could do this if you wanted to, and it wouldn't be more than a few days' work.

(Of course it wouldn't provide any value, which is why no-one does it).

If they are not public, that's obviously not the same thing, though you're still going through a lot more complexity than "here is this .war, put it on the server and reload the webapp".