I know this is an old thread. I searched before posting my own feature request for this so I'm posting my +1 for this old request instead of opening a new one. If this has already been solved then please point it out. Please read on, including my additional comments below:
this "feature" will lead only to camera crahes, stops and/or filesystem corruption...
I would take that risk. If the "write" LED acts the same like a computer HDD LED, the SD card operations are always conducted in a "low" energy drain phase (in Play mode or after taking a shot). The voltage drop should be far less than when i.e. charging the flash. I agree, that this is VERY DANGEROUS, buy the User has ALWAYS the CKDK's voltage lever indicator... and should react even better than the software 
I too would take the risk. In fact, I think CHDK
almost has what it needs to provide this feature:
In
Menu->CHDK Settings->OSD Settings->Battery:
There are already mV values for high/low battery voltages: This menu just needs to have a checkbox labeled:
"Use Battery MIN as the auto-shutdown voltage threshold [ x ]" .
For the paranoid, it could even popup
"Are you sure, this could lead to crashes or corruption if Battery MIN is too low".
I'm not sure if CHDK already has the "Shutdown Voltage" memory location figured out for all or some cameras, would be nice to know what is available there. Would appreciate it if any devs that could comment on the feature feasibility.
Thank you for your help!-Eric, KJ7LNW
