At the last GISAC meeting, I mentioned that we were working on similar ideas here at BYU. You can see our prototype at: http://www.lib.byu.edu/cdmmaps/
and at http://ds1.lib.byu.edu/maps/

There are basically two kinds of collections shown, historic photographs and a set of pre-war german topo maps. We're adding more.

These are working with ContentDM (sort of), and could easily be scaled to work with several remote collections at once.

The plan is to eventually open the code, but not until we finish the initial project, probably this spring.