id say no.
sad to say, i ran without that bolt too (did valve cover swap with some friends, they didnt put in the last bolt) and it leaked....
if it were on the side, id say run with extra silicone-goo, as im missing a bolt there and thats working fine on the side, but i doubt that would work on the back corner...
sorry man, the only way i see out of this is to remove the head... but the first step to that is going to be remove the valve cover. I would take the cover off and test and see if you can drill and tap it. it wont be easy.
i suggest taking out the motor mounts and lowering the motor as well.