Aside from potentially pragmatic reasons that are rather evident, has anyone seen any downloadable global IP to hostnames map database or flat file archives?

Yes, it would be large and by some standards enormous, but a compressed archive would make it reasonable.

There are various DNS history sites, so some form of this is already being done, but it seems as though there should be an open sourced project for this since this kind of information and history is invaluable for research and other things as well.