Hmm, older thread. I can't imagine the force on the trailer would be significantly different than the force on the drive sprocket. So, if it isn't slipping when you're standing on pedaling, then it probably won't slip with the trailer.

There are a number of non-axle attachments. The vintage Burley trailers used a seatstay/chainstay attachment. I have other trailers that simply clamp to the chain stays. The risk, of course, is wear to the paint.

Bike Friday uses a male airhose connector attached to the frame.

The Burley Travoy, and a few other trailers attach to the seatpost.