I don't think it would be a problem at all. If you look at my thread about my home made ladder, you can see the picture on all 210 pounds of me hanging off the ladder while the boat is on the trailer. I don't think you are going to get much more force than that on the stern, not matter what you hang off of the back. I carry that ladder and a 20 pound antique boat motor off the back of mine and it has never been a problem.
I was wondering more about items that are either screwed in or through bolted. I can't imagine most folks crawling down the seat tunnels to add additional backing material to the transom, so it must be fairly sturdy already. It does however appear to be hollow in that area from the sound of a knock.
The transom is indeed hollow, and must have backing plates, I used 1/4" oak. I cannot figure out how to post a picture to this group, so anyone wanting to see my installation, contact me at gregcoats1 at yahoo.com
When I first mounted my trolling motor I cut, routed and painted to match 2 boards to put under the clamp on the motor. On our maiden voyage one came loose, fell out and the motor wound up in the cockpit. We went back and retrieved the wayward board and I clamped the motor extra tight. That is when I saw that the transom was compressing because it is hollow. There is no easy way to get it tight with compression. Fortunately my motor will mount perfectly against the rub rail without the need to put any pressure on the outside of the transom.