from an earlier pic it looks like there is a fair amount of space under there above the playfield, I would certainly try to fit a coil sideways underneath the plastic with a lever arm to pull the diverter to one side and have it spring held the other way. if it happens to jam a ball in there the low power coil designed to be always on would be fine, and have it in your ball search routine if you have one.
not sure it that makes sense, or if there is really room as its hard to tell from pics. I'm also assuming the diverter is up against the rail at either end so the force of ball hits is taken up by that instead of the servo/coil/stepper.