Is a lithium ion button cell (LiR) big enough to pose a danger? I can't really find any tests.
Wikipedia claims several rechargeable chemistries for button cells including NiMH but most of the seem to barely exist.
Do you need more current than ML can provide?
For the type of applications I imagine, I wouldn't need more than 1mA of current...just enough to run an attiny-like microcontroller and blink a LED on occasion before going back to lowpower mode. It seems NiMH just doesn't come in those small sizes. I don't know much about ML, I'll have to look into if ML and LiR in the smallest coin size are safe enough. Actually now that I've thought about it more I might be able to just use a small supercapacitor for around an hour if I'm really careful with energy consumption.