My Guess? Not so good.
I think this is the first time we've heard this one. Can you post full details, year of the boat and such? It may be relatively easy to fix, but I'd play with it with the boat out of the water. Don't worry about the rudder falling out because, as I'm sure you,ve seen it's supported from above. The main concern with the boat in the water is leakage, which you say it doesn't. Once the boat's out for the season I'd pull the rudder and see if the tube will come out. If it does I'd look at epoxying it back in place after a thorough inspection if the surrounding area, maybe even taking the opportuntiy to replace the tube, possibly even designing a better replacement. I'll be fascinated to hear what Hunter has to say about this one! My bet is they have no tube and no drawings left to help you make one.Please keep us posted on this one!