If you want to attach something at a level lower than the top of the seat tube, then you might want to do it with a yoke that the hub can pass through. However, that may complicate wheel removal. I could see a yoke made in two pieces, one long that went to one side of the hub and back to the trailer. the other than bolted into the first part and went to the other side of the hub.

I don't think I would want to attach anything to the chain stays or seat stays, as those are usually the thinnest tubes in the frame.