My thoughts excacly, but there is something rewarding coming through for a long time faithful client. I love challenges and hate defeat (and the stench). I already told him "No guarentees and I'll see what I can do". I just don't know about urine, soap, and water, verses cassette tape
You can clean tape. It's something that restorers do when it's needed. I like the idea of getting a running but expendable transport. See if you can find someplace that still sells empty screw-assembled cassette shells. You could remove the tape from the peed-on shells and transplant it into a clean shell. Chances are the tape itself isn't damaged, but you could cobble up a cleaner from the expendable transport by removing the heads and holding a slightly damp cloth against the surface of the tape as you wind it through. You can play a tape wet so don't worry about not being able to dry it off - in fact that's one method of playing a gunked up tape when baking doesn't work.
If you get a couple of extra empty shells, you could probably rig one up as part of your cleaning fixture by cutting away enough of the front edge of a shell so that you can pull the tape out and loop it around your cleaning pad.