I can't answer that question directly, but I can give a side question's answer, and that might help you:
When you reinforce, all individual corps automatically try. I think that's a rules change from the original rules.
However, if there is a stack with a leader on it, whether due to a game bug, rule change, or whatever, it allows you to add that leader's corps counters one at a time. In fact, it REQUIRES they be added one at a time.
It may be possible (I haven't tried this -- I always want BIG stacks! :)) that one could succeed with a leader, and then only pick only a subset of the corps to actually add to the combat. Try it!