I don't have a master kill switch but I've considered it.
I have a metal bucket that I sometimes place uknown Lithium batteries in when charging.
Honestly, the only thing I worry about is that I may forget to turn off the soldering iron. I've considered putting it on a timer circuit..
I do have a Halon fire extinguisher in the lab as well as a dry chemical one. Hopefully I'll reach for the right one first if needed...