I feel for you - this is very difficult but not without the possibility of success. One of my mares had a sarcoid just below her ear and our vet did not recommend using Liverpool cream as this is was a very sensitive area with nerves etc. He injected it with a chemo drug and it worked wonders...
I wouldn't worry - even if we ride them the other 22.5 hrs per day we are on the ground with them anyway. Plenty you can do on the ground to make it interesting anyway. Important thing is you enjoy your munchy and love him.