Brian,
You may be able to drill a small hole through your top pintle for a retaining pin.
The first pic shows the small hole in the pintle and the second pic shows the pin I use to keep the rudder in place.
So far, my tether has been just fine. It's quick enough to tie a bowline in it. I take the rudder off all the time, because I dry sail the Harpoon, so a pin might be a bit more bother. In all honesty, I haven't had a problem with the spring clip holding the rudder in. In fact, this past summer, I used the boat to teach a capsize class for the club. I had her over several times, some on the way to a turtle, and one very definitive total inversion on purpose. I never saw any evidence that my spring clip was going to let the rudder come out of the gudgeons. Still, in a real capsize scenario, things are likely to be a bit more unexpected and vigorous, shall we say, so my little length of cord keeps me happy!
The spring loaded kick up design of a Ruddercraft rudder seems very nice, and will get the whole blade up out of the water. Certainly much better than my stock uphaul line which doesn't quite get it all the way out of the water until there are no people in the boat.
I'm surprised the rudder on my 192 doesn't have an uphaul line. I think I shall have to rig one this summer. There really isn't too much thin water where I sail at the lake, and no place particularly suited to beaching, but I like the idea of more control that an uphaul brings
Brian