The only way I can see to produce this information is to download each module and use existing dependency scanning modules (there are two or three of them, at least).

You could be optimistic and assume that all dependencies will be in the Makefile.PL files (or equivalent). Of course the reality is that they should be in there anyway, and when they are not, the CPAN-Tester should fail. So while it might be optimistic, it is more how it "should be" anyway.

That's a lot of work I'd rather avoid if somebody else has dependency information available.

(Also, if my idea overlaps with an existing project, I'd like to know about it too.)

I would email the CPAN-Testers list, as well as the perl-qa list (home of the Phalanx project) as well as the perl modules list(s),... that is if you haven't done that already. Other than that good luck and keep us updated as to your project, I think it could be a very useful thing for the community.