Hi there,
Is the urine alkaline? If so, a formula that you can get from Barbara called Hydrangea and Eupatorium should acidify the urine and help dissolve the stone.
If the pH is acidic already and you suspect CaOx urolithiasis, then I would expect formulas like Wei Ling Tang and San Ren Tang to help here, as mentioned in the short course. In addition, potassium citrate can help move calcium out of solution, allowing calcium to then leech from the stone into solution. By itself, it doesn't seem dependable. But with something like WLT (which would have the added benefit of settling the cat's stomach), it might.
Another option, which I don't think works as predictably and for which there is less research support, is to use a 'stone dissolving formula' like San Jin Tang. It contains chicken gizzard as one of the active ingredients, which Chinese medicine credits with stone dissolving powers. If the cat would eat chicken gizzards, you could also feed them to it and see if it helps in some way. Wei Ling Tang has good research support behind it, however.
Steve