As I think about this more, I’m wondering if what I want isn’t something like this:

As a Free Action, you can impose Simple Tactical Oppositions, generally against a very Focused set of targets.   Good examples are “I avoid him” (Athletics), “I stay between those two” (Tactics), “I keep out of line of sight of the archer” (Dodge). The target gets a reflexive opposition to this; if they win, they ignore your constraint.   If they lose, however, they can still operate past your constraint; you just get the result dice from the Tactical Opposition roll to apply to your contest (so if you were trying to stay out of line of sight of the archer, and won by 6, you get 2 bonus dice on your dodge).

But then there is complex opposition - where you are burning a Full Action to do something.   Fundamentally, that isn’t overridable.   If you burn your full action to defend someone else, the laws of the Universe now require anyone to beat you up before they go after your protectee.  The more heroic, the more you can get away with.   A complex action is constrained to an activity, but not to a target; so you can defend person A, against anyone.   Trying to defend a lot of people is harder; and may be a custom ruling.

 

Still need to work on the constraints, right now it feels like, “Player proposes, GM disposes.”