Radical! is a very cool and unusual game. I would love to see that being tweaked for better rules in (multi player) competition usage.
However. Individual locks made-progression towards multi-ball is not possible on games with no auto-plunger. The best it can be is with first-in-first-out one ball at a time lock mechs like Funhouse.
You can prevent lock steeling. By resetting some progression before each start-of-ball. But that is tricky. It involves ejecting locked balls and waiting until they return to trough before serving up next player. I certainly foresee a hazard or two. With flaky switches, inconsistent sequence of events, points awarded from ejected balls, draining (locked) balls ending ball-in-play for the next man up etc.
At the top of my head I cannot think of any game that does this. Or wait, Doctor Who?
Personally I do not see it as that big of a deal on 2-ball’ers. As long as the “lock lit” status is not stolen as well.