Thanks for the well wishes, everyone! (And Stryker, thanks for the link - I'll check it out!) My family appreciates it.
Unfortunately I don't have a positive update today.
Lucy went back to the vet for a second round of unsuccessful tests. At this point there is no clear next diagnostic step. There's always another test possible, but since it would become increasingly invasive and would be essentially at random, the vet told us that in her professional opinion the stress on Lucy's system from more medical procedures would outweigh the small possibility of more tests turning something up.
We've taken Lucy home and are trying to make her comfortable. We will continue her medications, and I've been syringe feeding her to make sure she's getting some nutrition. We hope she will take a turn for the better and pull through, but the vet thinks that most likely she has reached the natural end of her lifespan, which may have been shortened by a tough early life.
Thanks again for your support. It means a lot.