Yep - interesting idea.
The ones I have used were for the designated purpose and either connected by wires or plugged into the domestic electrical mains; the latter were incredible, very sensetive with a range more then adequate for the Angler. My wife informs that as David infers there are now battery operated devices. Blimey, what will they think of next!.
Only problem I can think of - might be a problem with 'bleed over' (channel interferance from other similar devices), if the idea caught on!, image stormin' out of the bivy in the pouring rain only to see the guy a 100yds away into a good un & your indicators motionless!.