Hereford: I wrote a number of posts here outlining how I usually free up my O23 stuck centerboard with a metal rod. However, I was referring to my '79 Oday 23, which has a completely different configuration than your '73. The later models of the Oday 23 (the Mark II version) control the centerboard with a pendent that runs up through the keel, under the cabin steps and out into the cockpit, where it comes out through a hole in the bridge deck. There are no cranks or other mechanical devices; the pendent is controlled just by hand. When adjusted up or down to your liking, the pendent line is simply cleated off on the bridge deck. When usticking my centerboard, it is a simple matter to push a three foot long 1/4" metal rod through the bridge deck hole down past the pendent line until it touches the stuck board; apply the appropriate amount of pressure, and the board is free. I believe the fear of puncturing the pendent line conduit comes about when that conduit is made of flexible hose. On my 23, it appears to be made of some solid material, so I am not that concerned about a puncture, especially because I am very careful when putting the rod into the conduit. I do remember reading about someone who did puncture his conduit, although I think he had an Oday 25, not a 23. It can't be a simple fix as the conduit is relatively inaccessable. I would guess you would need to haul the boat to fix something like that.
However, on the older models of the Oday 23, the pendent line is lead into the cabin through the support post for the table, where it is controlled by a crank. I have never taken one apart, but I bet you can remove the table top, exposing the interior of the support post. It should be a straight shot down to the centerboard, with no flexible hose to worry about. A metal rod should theoretically work for you too. However, keep in mind that you can't really exert that much force with one of these skinny little rods; certainly not enough to free up the board if it is rusted to the keel. The method works fine if you have some barnacle encrustation or a stuck shackle, but I have grave doubts that it can do much good if you have a big problem. I would be more concerned that you could damage the centerboard by forcing the rod down onto its edge if the board is really stuck.