It is an interesting idea, and I think you could make it work.
I might tie it more to the attacker's total OB rather than OB/DB split, or else it will be a harder maneuver to disengage from a feeble Wizard with a dagger (OB total of 10 but all 10 spent on attacking) than a skilled swordsman (OB 150 but reserving 10 of that 140 for parry).
RMU has a section on Withdrawing that was added (imho) due to player demand for a bit more clarity on how one withdraws from another character's zone of control ('combat zone' -- another new addition), though still nothing quite so well defined as D&D's Disengage. To fill the gap, I've added a D&D style disengage action to my own games, which is just a 1 Action Point (25% activity) with no roll required, but I allow attackers also to 'Press' (reserve 1 AP or 25% activity) so they can pursue the person disengaging.
I think JDale also uses opposed checks for this sort of thing, so that might be an option as well.