I know you're feeding him a high content hay diet to help keep his teeth short and this is really important with root issues as well. But when he has been bad in the past have you noticed if he's better with soft food? From my experience with Grim, he wouldn't touch hay after his roots got very bad but he would still eat pellets and then when they got worse he had to have soft pellets. He could still manage things like broccoli so I think it might have something to do with how they chew it that was causing pain. :?
I think the x rays are worth doing if you're covered by the insurance, but there is no cure as such for root issues, just pain and food management. In the end with Grim I gave up trying to keep his teeth short and just gave him food he was comfortable eating, so all soft food. This kept his guts going and helped avoid any stasis issues. He just has soggy pellets available at all times. he had hay as well but it was just too painful to eat. He needed dentals but they got further and further apart. It seemed after time that on an all pellet diet his teeth stopped growing. I think it was Amethyst that noticed this as well with one of her bunnies. It got to the point he was having a dental every eight months and for a rabbit who has zero hay that's quite incredible. Obviously after that his teeth started to rot and fall out so now his dentals are much less frequent that that.
I know you've said in the past that he's a good hay eater which seems to indicate the opposite of what I've had experience with.