ok how about this:
When toggling reactions player can exclude enemies. "Yes, I want to perform attack of opportunity but not against THAT enemy. Maybe also not against that one". With a clear action cue, and presumably smaller engagements this should be enough to have a pretty good control over to whom characters will "react to" and with what.
Exclusion would be per-reaction. So you could not include one enemy in "attack of opportunity" but have his tagged with something else.
