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?