The only thing I really had to work out in the code (sample here) was the timer settings to use - against everyone's good advice, only Timer0 seemed to work reliably on the Flora.
The biggest part of this project was actually figuring out a comfortable way to carry the battery on my body, without wires getting in the way or any weight being carried by the choker itself. Pictures below of the result, a somewhat elaborate harness made from elasticated ribbon and o-rings. For a bigger battery, plain elastic would work too - the main thing is that the battery is held in place by multiple straps, so it moves with the body and doesn't swing like a backpack would.
Testing pic. Not pictured: various testing activities attempting to raise my heart rate for display.