I've got good news, for once. Scone had his follow-up blood work and ultrasound today, and the results were fantastic. His prognosis is now "good".
The BUN and creatinine levels are back down - BUN went from 63 to 32 (29 is normal, but the vet said 32 is fine) and creatinine went from 5.0 to 2.0 (normal is .5-2.5). The ultrasound showed that the kidneys and ureters were almost normal in size, and the stones were not visible at all. The vet said the radiologist was amazed at the improvement. There's still some mineralization in the right kidney, but it appears to be of a sort which isn't going anywhere and isn't dangerous. He's regained a few ounces of the weight he lost - still under 5 pounds, but not by much.
I'm supposed to reduce the Adequan to once a week for the next month, and if he's still doing well it will be cut back further to once every two weeks from then on. The vet said to give sub-q fluids twice a week for a month, also. We have an appointment on December 23rd to see if everything's still OK, and if so they'll discontinue the sub-q.
The vet's report ended, "Scone is a very sweet rabbit!" I agree.