The reason I keep bringing up chain stays is because there are lots of existing chain stay attaching systems already. I pulled kids in kid trailers using these and they work very well, keep push pulling motion down low.
I could look into it. I used to have a trailer like that when my son was young. We also had a tag-a -long which was wonderful to use. I actually went out and bought a vintage Diamondback MTB just for that purpose and built it up as a bomb-proof tourer.
When we had the tag-a-long, it took some explaining to get my son to stop trying to steer the bike by leaning, wiggling, etc. I was lucky to have it attached to that DB. It is one solid bike. Heavy bike though...