there is a power state that could be observed in /sys if I recall correctly.
so it would really be something like "the amount of time the device has been requested to go into a non-standby state since the last charge"
but to the end-user it would be much the same thing.
Again. as I say... assuming the event could be neatly "watched" this is not a tough thing to write...
the On Screen Display would be the trickiest part I think... a report on demand? or a little OSD overlay... hmm..
anyways. those are my thoughts. this could be done for sure.