Looking him over again, and consulting with the oncologist, the vet realized that the surgery would have to be much more radical than we'd discussed, to the point of possibly removing part of his spine as well. Also there would be no muscle left as a cushion (the tumor killed a lot of the muscle there). She now feels that the odds of preventing a recurrence would be the same or greater simply continuing to dose him with prednisone than with surgery, especially at Dinsdale's age (around 13).
So we decided not to go through with the amputation and instead keep giving him the prednisone. He's home now, active (he was scratching at my bedroom door this morning for the first time in a long while), his appetite's back up; as far as I can tell he's comfortable (though I'm not sure just how well I can tell).
So. We'll just see how long this goes...
Thank you very much for all the GoodThoughts and prayers and support.