This seems sensible and sane to me, though I think I would at least say they can't claim on boards they're rephacked -0.7 or lower with--mainly because bounty hunters do hunt criminals, specifically, and anyone claiming on a board of a faction they're hostile to would be...silly, to say the least.
Or, perhaps, instead of it being based on rephack it could just be like bounty hunters and UC provide a screenshot proving rep when they claim, but they don't have to register first. That way you can circumvent the whole registration process for those IDs and you ensure that they're not claiming for their literal enemies.