You could cut enough off the bottom of the rudder to make it no deeper than the keel, then glass an end-plate to it (think wing-keel) to regain the "bite" on the water. Phil Bolger used that idea on many of his ultra shoal-draft designs and it is supposed to work very well. It may even work better than the original rudder design. I would not go wit ha kick-up rudder, seems pointless to switch to a more moving parts, complicated design just to overcome the 3-6" extra depth of the rudder compared to the keel. On a true centerboard boat (instead of a K/CB) I see the value of the kick-up rudder, since the CB retracts completely into the hull and having the rudder sticking down 24" +/- below the boat defeats the draft advantage of the CB. On an O'DAY 20 or 22 (1979-83 K/CB) I DO see the advantage of a kick-up rudder and can't understand why O'DAY never made that feature standard (and never even an option on the 22, only the 20). The 22 with the K/CB draws about 14" CB up, yet the rudder sticks down like 18-24". But for the 23 or 25, not worth it just to "fix" the rudder being 6" deeper than the keel. I'd also try sailing (or motoring) with the CB lowered about 8" so that hits before the rudder giving you a chance to turn towards deeper water before the rudder hits. O'DAY made provision for inserting a pin in the upper pintle to prevent the rudder from lifting (or floating) off while underway, using a pin like that should help prevent loss of steering if you did touch bottom.
I've attached a couple of pics of the rudder on a couple of Bolger-designed catboats to show the end-plate.