This would, IMO, be the best approach. We already have players (Amazon, Nokia, etc) that would certainly be interested in having such a layer that they could apply on top of their services.
Then, having an open source community contributing with open implementations could help filling the gaps to make life easier to those players (and probably lowering the bar to allow new ones to join) and, who knows, to make it possible to have an AOSP device fully compatible with Play Services APIs without the need for gapps, probably just using some private server or other custom solution. Maybe I'm misunderstanding something somewhere, but that seems to be the goal of this project. If it is, then I really hope for its success.