@rlkeith, do you have room for 2xAA batteries? They are about the same footprint as the Xenon.
Two (L91) AA batteries (direct drive) would give you 3.6V when full, down to 2.7V when depleted.
I’m guessing these will be a decent baseline for battery operated direct-driven Xenons once we have sleep modes, assuming 3.6V isn’t too high.
The L91’s performed great for a Boron in this post (see Trial #2), but using 3xAA and the Boron’s on-board regulator.