Very useful information, E-wan!
I usually make my own trailers and braze-up a custom hitch for each bike, designed to go beneath the left-hand side of the rear hub QR, and designed with a ledge so it won't clock around the dropout under load and inertia. Works great and a quick project. Just thinking...if a commercial hitch for a given trailer didn't happen to fit, this would be a simple/cheap project for any machine shop or framebuilder, and avoids any modifications to the existing frame.
Best,
Dan.