I think that generally there aren't enough corps involved in most battles to make this work. Also, the combat cards would become weird, and the battle table would need to be totally thrown out and a new CRT would need to be come up with. Too much work for not enough payoff IMHO...
There are a few options that could make it work, as follows.
Minimum Battle Size:
Off-map tactical resolution only occurs if the battle involves at least 3 corps on each side and one or both players wish to fight the battle tactically.
Translation of Corps into off-map tactical units:
Say three French corps attack four Austrian corps in Bayern. The three French corps might translate into three corps commanders and a dozen or so battalions, including separate infantry, cavalry and artillery battalions. You would need figures or additional counters for these. The exact composition of each corps could be based on a standard template with some variation between battles.
I know this is all very vague at the moment. Just thought it might spark some interest.