As it turns out, Just van den Broecke has already done what my quick at-a-glance feasibility study set out to investigate, and integrates Open Streetmap data on top of Google Maps.
He plugs in the Open Streetmap imagery as a third party Web Map Server (WMS) into the (not yet publicly documented, though still usable) Google Maps API, in the same fashion the Google Maps native "Hybrid" view works, with a layer of transparent images stacked on top of the Satellite view, as I did. Read his blog entry on it for more information.
Van den Broecke has done a few other WMS + Google Maps mashups worth noting too, where he features NASA/JPL imagery (I presume that it gets those interesting looks due to the NASA/JPL WMS not working with the Mercator projection that Google Maps uses) and Catalunya maps.