You can do something like that right now without even modifying the client. Just connect to someone's Bitcoin client over a LAN and send them a transaction.
I'm trying to think along the lines of the Dash7 networking, which doesn't use sessions like wifi, so there is no LAN per se. Even though IP6 is supposedly possible over an ad-hoc Dash7 net, I doubt that would be the ideal solution. Something a bit more native than just IP over Dash7, and as I already mentioned, Dash7's topology is already very Jabber friendly. I don't doubt that a Bitcoin protocal to run natively on Dash7's stack is also possible, perhaps prefered, but I was also looking at cloaking the Bitcoin traffic into the (assumed) Jabber traffic, making it more difficult to 'sniff' out Bitcoin users in a crowd. Mostly because I don't want to add a reason to make cellphone theft profitable, not because I'm particularly concerned about govco snooping; although that would be a positive side effect.