Seems a pretty good idea! Though I'm not entirely sure if such a compo will draw many (enough) partcipants.
I wonder how many of us have the means (and knowledge) to create a cross-platform game.

I like the idea of a compo, but given the fact that it'll be our first, I think we should aim for a more broader audience, and see how that one develops. If there's a lot of interest, chances are a compo with more demanding rules, like yours will get some as well.