Good plan, saves a bunch of space and eliminates changes due to mechanical forces (although I doubt that would be a problem)! Did you consider the NRF24LE1 SOC solution? From the data sheet it looks like the whole thing only takes 20 or less ma (I think a pro mini is up at 200 ma or so). They seem to say it can run off 2032 type button cells (and I just discovered they make rechargable 2032s!). Its hard to tell with ebay modules til I get some (I ordered a pair to have a look at) but it looks like the board is 3cm by 2cm which is even smaller (and it is both the radio and the mcu so one less board saving even more).