Maybe you could try sitting down on the floor, with your legs in the position that they would need to be in for a roll. Roll backwards onto your upper back/shoulders and then forward again to the previous position. As you get more comfortable, throw yourself forward with a little more force until you are able to send yourself into a standing position. Then, sit down and roll back onto your shoulders again.
You can do this by yourself, with little risk of injury. If all goes well, you'll train your legs to stay where they should be, and train your body to come up on one of your hips, instead of both.
Please let us know if any of this stuff works for you.
Enjoy your "rocking chair rolls". Good luck.