The Joule Thief was my first thought, too. Figured OP had probably been there done that, though. If not, well there ya go.
BTW, I'm 100% all for doing something for the heck of it. Not every... scratch that,
no aspect of a hobby has to be rational. For that matter, I always try to find less demanding applications for dead batteries too. It's not about the equivalent grid cost, since most of these devices can't be run from the grid anyway.
Incidentally, my bathroom wall clock has served as a battery tester four times in the last couple weeks, going on number five... as its time will be correct in about 8 hours, for around one minute.