Intuitively, given:
leds are low volt
li-ion/lifepo4 canisters are lo volt
multiple canisters are a hassle (balancing/housing)
cars have 2 lights on 2 front corners & it works for them, so why not bikes -2 lights widely spaced?
THEN:
isnt the ideal a ~cree pack that runs from the native voltage (3.2v-3.4v) of a suitable lifepo4 cannister like the 38120?
(a 10ah cannister is 33 watts for an hour, .33 kg, my guess is you can recharge from usb), methinks i saw 12/13 ah cells also
lovely threaded contacts on each cell
e.g.
http://www.batteryspace.com/lifepo4...gerate32whwith6mscrewterminal-unapproved.aspx
disposable cells should be fine for rear lights small needs. not worth wiring them in (3.2v wont like distance)?
I have 2 x 2$ 2xAA on back, last forever but if one goes flat or has problems, always have one for now.
one trap i have found, is if you get greedy about cell size in cheap tail lights, is they shake loose on bumps. So Maybe opt for aaa over aa? Ideally the cells should be housed facing up and the tail light pointed back.