Further follow up:
Not so good.
I rode with the Cyo on a couple of 200's and on many commutes, and was rather satisfied. Last couple of days I've had trouble with it refusing to light up properly (comes on super dim, like a cheap LED blinkie).
I thought perhaps my old Ay-Up lipol battery might simply be dying, so last night I tried swapping it with another, but no better.
Fair enough, I thought - I can try this with a 6 cell NiMH (7.2v) RC battery. Again, no dice, so it looks like something in the light has broken.
However, I was able to get it to light up easily at full brightness through the tail light connectors. Gambling that the tail light could handle it, I wired both lights in parallel (by poking the battery wires into the tail light connectors along with the headlamp wires!) and WIN. Back in business.
However this morning by the time I got to work, the Cyo was not lighting up at all, although the taillight was chugging away quite happily. So either the Cyo is fully broken now, or the voltage of the battery has fallen below some kind of threshold required to get any light at all (which sounds plausible to my non-electrical brain...)
Either way, looks like I might need to take it apart and wire up the LED directly.
Does anyone know what the design voltage of dynamo taillights are, please? My google-fu is weak (or I'm not hitting the right sites) and I like the toplight line plus enough to want to preserve it...