Done it
I've had to re-insert the rudder twice while in the water on my 1980 H27. It failed the same way yours did: crevice corrosion at the stainless steel junction where the glass created an oxygen starved coating on steel. You don't need scuba gear; you will only be under water for a few seconds at a time (and yes, it will take a couple of tries to get it in - budget 20 minutes). The rudder bottom floats, while the top sinks, so to counter this you have the ride the rudder like a stick pony, keeping the shaft pointed up. Submerge the rudder in this way, and, keeping it perfectly vertical, align it with the hole. The fit is very tight, so don't try taping over the top to keep water out of the shaft - it will jam on the tape (experience speaking). Archemedes will keep it in place, so slowly let it float all the way in to the bearing.Hard as it is, its better than trying it on land; you would have to dig a hole in the lot to get it to fit!