Hmm, interesting question.
Already thought about it too, if it is even possible with onboard methods to get a Multiplayer in LAN (with or without Internet) done.
It might be a bit tricky, dont know if HTML & CS are powerfull enough to check for your Environment (check if you are in a LAN or connected via the WAN). I think it might be possible, but dont know for sure.
It migth be possible (in LANs) with Scirras Signalling Server (you can get it in the store), if you get it done, running it from the app/game directly and check the net, if there isnt already an another.. somehow it has to be possible, but how?