I can understand the problem. It does seem that some senior bunnies have an issue with keeping weight on.
I think that maybe I would do some research into the ingredients of other rabbit pellets, as if it's just one ingredient specific to the one she has, you might find that she could tolerate another pellet. Problem is that it might mean experimenting with several different ones and it would become expensive, although I guess if she rejected them, Boo and Joey would step in.
I would consider different options, maybe one based on hay with foragey stuff in it. Another option would be to try a junior pellet, which would have a higher calorific value, so she maybe wouldn't need to eat many.
I think that maybe I would do some research into the ingredients of other rabbit pellets, as if it's just one ingredient specific to the one she has, you might find that she could tolerate another pellet. Problem is that it might mean experimenting with several different ones and it would become expensive, although I guess if she rejected them, Boo and Joey would step in.
I would consider different options, maybe one based on hay with foragey stuff in it. Another option would be to try a junior pellet, which would have a higher calorific value, so she maybe wouldn't need to eat many.