Hi Richard,
Only just seen this post, so here are my thoughts. At my sailing club in Poole there are 4 Super Seal 26s, and the oldest is a Baker version and has had the rudder blade fail twice, in exactly the manner you describe. The first blade broke off completely and was replaced with (I believe) a second hand blade which subsequently failed in the same way. Both these blades looked to be a thick(ish) grp skin, with a foam core. The Rudder on my Super Seal 26 Evangeline (No. 97 and the first built by Parkers) is definitely a hardwood core, with a very thin skin of fibreglass over the outside to protect it. A couple of years ago a large part of the sheathing (where the blade sits in the rudder stock) had become detached from the wood so I cut it away. cleaned and dried everything then stuck it back down with epoxy. The basic core/srength of the blade comes from the wood, which is a dark red colour, so I would guess is either mahogany or sapele.
As Jan (above) says, the construction is very similar to most dinghies so I don't doubt a wooden dinghy manufacturer should be able to help you out. I can't see that the fibreglass skin provides anything other than ease of maintenance, and protection for the wood, so I don't really see why you couldn't make a new rudder solely out of hard wood and varnish it. Any good carpenter ought to be able to do it.
The two Evolution 26s which I've sailed on, which are very similar boats to the Super Seal, both had varnished wooden rudder blades, albent vertically lifting rather than swinging.
Let us know how you get on.
John
John Guess
SS26 Evangeline