If the rash is caused by an allergy to the cat, there is really only one solution, you know! Soapwort, aloe vera, chamomile, St John's wort, lemon balm, houseleek, chickweed are other herbs which can ease an itch when applied externally. But they'll only give temporary relief - until the source of the problem is gone, it will be a continuing problem.

also, a poultice of Plantago major works... but like Daisy says... if there is a cause that is not removed... that's only a band-aid for a problem that will continue.