Skip the regulator, run the whole thing from 3 AAs, or from a 3.7-4.2V Lipo, and slow the clock down to 8 MHz.
This remote control is similar. Uses a 8 MHz promini that goes into powerdown sleep mode, wakes on a button press, reads the keypad using Keypad library, sends it via 433 MHz Tx with Virtualwire library, goes back to sleep.
Debugged with 3 AAs, runs now from 1000mAH LiPo under the board.
Recharge controlled by MAX1811 (not shown, goes in the empty 8 pin socket with a couple Rs & Cs), recharges from 5V source (which can be USB port).