It looks like most of the Taglights used the X-light Micro controller. I have seen the processor on those have problems if they are turned on and off rapidly. Sometimes it only takes a couple of cycles to cause the problem.
The processor problem that I have seen, will cause the light to only work in momentary mode, ie. they only stay on while the switch is pressed down and the variable brightness function won't work. The fix for that problem is to remove power from the circuit for a few seconds.
Tough to do if the whole thing has been potted in silicone though and unfortunately, letting the battery run down wont do it either as the lithium polymer protection circuit will put the battery in a low power mode and it will still output 3.2v to the light control circuit (if the batteries he used have protection circuits that is). I guess if you let it sit long enough, the battery really would run out of power and not supply anything but the protection circuit might keep the battery from accepting a charge at that point.
That being said though, I'm not sure if that is what is going on with your lights. I have never seen an X-light Micro controller "stuck" in the on position like that.
The above information probably isn't much help to you but basically, I think the silicone would have to be removed and the circuits exposed to have a chance at fixing them.
