Pretty sure Adrian had put in a filament alarm in one of his revisions. I haven't tested it to be able to compare to your idea though.

I have been using Adrian's firmware and filament alarm for a few months now.

http://www.soliforum.com/post/27993/#p27993

This runs to a microswitch and when it goes open causes the printer to @PAUSE and saves the last position. You still have to run the M600 (or M601 I can't remember) command then hit continue to get it to return to the last saved position and start printing. This was also built into the Controller so you only have to hit resume print on the panel if you are using it for standalone.  I have my clip-on micro switch adapter on the post above.

I don't know what pin the PrintrBoard uses. You might have to futz with that still.

I printed out a little box that incorporates a micro snap action switch to check if the filament has run out/broken. A very simple application would be to wire the switch to the RESET pin and the printer would then reset itself (turning off the heaters) and thus stop a condition where your filament ran out.

But this is not very useful all by itself. I have been looking through the firmware and was pondering adding a routine that would check this switch periodically. If the filament ran out, then the routine could power off the extrusion heater and move the Z axis down a little. It would wait for new filament to be inserted into the box and then re-enable the extrusion heater while disabling the extrusion stepper. You could then pull the remaining filament out of the pinch wheel and then insert the new filament line. At this point, it would be necessary to add a button to then continue the print where it left off (by enabling the extrusion stepper, move the Z back, and then continue on).

Trust me, I do have a good sense how much trouble this would be to write. But I think ti would be worth it for 2 reasons:

1: Currently, I just estimate how much filament is left on the reel and keep doing small jobs until it is down to a few wraps left. It would be nice to just print out something big and use it all up.

2: You could manipulate this routine to do multi-color prints. Cut pre-determined lengths of different colors and as each run out, put a new color in.

The only thing that I do not have a handle on is that I cannot see anything in the current Marlin that tells RH to pause from the motherboard side. If I find a way to tell RH to 'stop and wait' while the routine takes over, I can make this a reality in a couple of weeks.

If I can get this idea working, then I would tackle a slightly more useful version of this where the filament would turn a wheel instead. If the wheel stops (filament out or nozzle clogged) then do somewhat the same thing and maybe prevent some disasters.

Anyway, does any of this sound useful?