For completeness, here are the circuits I ended up using. The flasher design is admittedly ham-fisted; I built it in a hurry.

The battery monitor is a bit nicer and power-conscious, but if I had it to do over again, I'd increase the hysteresis a bit (maybe drop those 4M resistors to 2M). I did the prototyping with the bike lights before I added D2/C1/C2, which effectively cuts the hysteresis enough the voltage swing from the flasher is enough to cause the indicator to flash between red and green a few times with the halogens on high. Oops. I'll say that's a feature, not a bug... yeah.

In addition to the obvious nightime usage, the flashers are great for helping me stay visible on misty mornings, going under bridges, etc. in the daytime.

-JAB