I don't know if the FSRs are sensitive enough to actually detect the weight of average prints.
I was actually thinking more for all printers in general, not just FLUX specific, which is why I was thinking just a setting up a simple optical gate type setup on any open side of the printer. Could be done with fairly small bubble housings to hold the sensors, any trigger would trip a simple gcode dwell command. A few different ways to skin that cat.
The print stuck to the head, you are right, that's why I tagged you in, the thermal detection I think would be too difficult, there just isn't enough sensor information to detect it that way. An end stop ring type switch, I think I get what you are saying, and that might actually work. Offset it 4mm or so from the nozzle, so about an 8mm ring on a pivot switch. Enough leeway that it shouldn't accidentally trigger, but close enough to shut things down before it wads up a hot-end too badly... Nice!
BTW, this counts as prior mention, so Open Source