Well, after using the trailer a couple times, I became a little concerned that the surge from the old spring-based hitch might become a problem as the weight climbed (eg, 20-40lbs of rabbit litter). So I decided to try and build a hinged universal joint that would do the job. You can see what I can up with below:

I settled on shelf brackets (which are nice and rigid) mounted to a piece of electrical box cover for the hinge on the bike, and a pair of straightened brackets for the trailer arm hinge. The result seems to work fairly well, demonstrating noticably less surge than the previous design. You can read a bit more about it on the Bicycle Trailer page.