I've never had that problem, but what I would try if they were my negatives is to take the least important negative, and try immersing it in an acetic acid bath. The reason I suggest this is that cat urine is a very strong base, and the acetic acid should neutralize it.(posted 8621 days ago)It might stain the negative, as when you mix an acid and a base, you get salt and water (if I remember my chemical theory). A thorough wash and immerion in a wetting agent, such as Kodak Photo-Flo, would be desirable before drying.
I recall reading in a newspaper column advice for a person who had the problem of cats spraying on the siding on his house; it was suggested that he try a 40% solution of acetic acid (2 parts glacial acetic acid to 3 parts water) to neutralize the odor. The stop bath suggested for general photographic use is a solution of about 1.3%, which probably have little or no effect on the cat urine; I would hazard a guess that a 28% solution would be the best place to start. If it doesn't work, try a more concentrated solution. Be sure to take all the usual precautions, such as eye and hand protection when handling acid, and don't forget to add the acid TO the water, not the other way around!
Let me know how you make out.