I decided to make an alarm clock out of this little project. Here is what it looks like:
First I cut the battery holder at the bottom to give the clock an angled standing position:
The remaining single battery compartment holds a 700 mAh AA sized LiFePO4 battery. The cell voltage of 3.2 V is perfect for this application.
I also changed the schematic a tiny bit and added a white LED to the buzzer pin. In the dark it can be used as a handy flashlight.
The firmware from my previous alarm clock  was adapted. Please find it attached to this post. Due to the larger flash memory size of 8 kB of the MSP430G2452, the wake-up tune is a bit more sophisticated. (Taken from the SHizZLE demo  starting at about 1:15.)
In standby mode the clock draws about 2.6 uA. Displaying the time consumes about 18 mA (auto power-off after 4 seconds), using it as a flashlight by having the white LED turned on also draws about 18 mA (auto power-off after 1 minute).
The rechargeable LiFePO4 battery I used is rated having 700 mAh @ 3.2 V. Doing the math gives the following theoretical values:
Standby: 30 years
constant on flashlight/displaying time: 43 hours
I guess the real limit is the self discharge of the battery, but LiFePO4 batteries perform quite well in this respect.
So I expect a few years of regular usage before I have to recharge it :)
A short video of the alarm clock in action is available on YouTube: http://youtu.be/UG2Iyw69h-E