Okay, since we went away from implementing low voltage alarms anyways...
It's about pinning down the location of your lost quad:
For the FPV case in which you still have an "OK" video signal... Maybe some of the following works:
Let's imagine you have a diversity receiver with a helical (directional) and omnidirectional antenna - you should be able to figure out the rough direction of your quad by turning around until the receiver switches between omnidirectional and directional. If that's not enough - a single receiver with RSSI measurement + directional antenna should do it - just run into the direction with the highest RSSI. Since I don't have a helical antenna here yet I can't try that right now.
Regarding RSSI: At least the FR632 diversity receiver has pads on the PCB where you should be able to simply solder some wires on. Maybe attaching something like this voltage meter from BG could be cool.
Weight: 0g on the quad.