When I used to go to a gym, I would see these guys doing pull ups wearing some belt that they attached a weight to. I don't know if that would work for you.

In the gym, I used this cool machine where you would stand on a bar and then put your hands on a pull up bar. The bar I was standing on could be set to push me up a certain amount and help me do a pull up. It's kind of the opposite from what you're looking for. I can't imagine being able to do a pull up ever, but even doing the fake ones where I was getting an upward push felt pretty neat. I'm glad you're doing well.