It's not that the hex key doesn't work, it's that it's too much work. 1/4 turn at a time is all there's clearance for and it's annoying.
I'll see if I can find a hex socket that size for my 1/4 drive torque wrench -- I have a set of 1/4 drive hex sockets, but it didn't include one that big. BTW that's a lot of torque for a 1/4 in torque wrench.