Well, I've discovered that although PAT dogs cannot be raw fed the other two main organisations - Therapy Dogs Nationwide and Canine Companions - are quite happy to have raw fed dogs working for them. TDN posted the following in their FAQs.
My dog is RAW fed, can they still become a Therapy Dog?
Yes. We allow raw fed dogs to join TDN. There is no evidence to show that raw fed dogs prove a higher infection risk than kibble fed dogs and we believe in feeding your dog a complete balanced species appropriate diet.
The Royal College of Nursing (RCN) produced guidelines in 2017 stating that RAW fed dogs should not visit a hospital environment but in practice, many hospitals we visit have made a judgement based on their own experience and have not excluded us from visiting. That said, an establishment may ask that they do not have a RAW fed dog in which case we will respect their decision and help you find an alternative placement.
www.tdn.org.uk/