Not the easiest thing in the world to do, but I've done it. So I know it can be done.
I faced the exact same dilemma: Just got a new top end together and was getting a leak at the pvalve. I DID NOT want to have to disassemble that puppy just to get an o-ring in there. So I took a stubby 10mm wrench (I think it was 10mm) and got it loosened. You only get like 20-30-degrees of motion, so you have to be patient and expect it to take a while. Once I got it loose and got the linkage and water pump idler out of the way, I could get my monster phillips screwdriver on there at an angle to back it out. Isn't that long of a bolt.
Maybe an open-end wrench socket attachment on a 1/4" drive would work better? Maybe find a cheapy wrench and grind it down to fit better?