Perhaps it could work as some sort of bonus units, that can not be repaired. After all it wouldn't make sense to keep captured french tank units up to strength throughout the war - they'd run out eventually.
IIRC, PG4, which had captured units, didn't place such restriction, and it's unneccessary, because it makes the units useless right on the spot.
Repairing them can be explained away by requesting other units of the same line captured elsewhere on the front.
Chances are, before long, the unit will have to be replaced by a more modern one anyway.
At the very least, the game options should be expanded to allow scenario designers to hand out any unit as a reward and to possibly determine the maximum amount of those units the game will allow - like the three elite units limit, that is hardcoded as of now, isn't it?
Shouldn't be too hard to code.
Btw., anyone knows what happened to Rudankort? He's been strangely quite for a week or so now...