"Light theft is a big problem here, so I want to avoild a battery powered removable light."
Asked and answered.
All of mine are removable. That, perhaps, would change if I commuted by bike and not tele---. My '59 Sports uses the front dyno, and it's not something I'd depend on to see. I have had two bottle-type, and I didn't like either one, but they worked, valued as much for the taillight as the headlight, because for me, being seen is the goal.
"I’m trying to help a friend power some lights on the cheap."
Lights to see, or be seen? That kind of drives it. I've got cheap round lights from China that are rechargeable, micro-USB, and they're fine, but under-powered if I want to illuminate my path vs. just be visible to others. I've also got a MyNewt that works great but is a PITA with iit's separate battery pack. I've also got a Garmin Varia that is the bomb.
The inevitable conflict always comes with most devices when performance is required on the cheap.
We want to play, not to pay. Human nature.