Thanks for all the good discussion.

I don't understand why flow separation above a full-span flaperon
would cause you to lose aileron control everywhere in the speed
braking region. Let's say you're at 45 degrees flap, and the flow has
separated on top, but you'd have plenty of pressure on them below. So
I'd guess the force on the control surface might be at 45 degrees or
so above horizontal. It seems you would still have some good aileron
control because raising or lowering opposite-side flaperons would
still change that force a lot, with its vertical component. It seems
the bigger problem might be yawing effects that would come with it?