rjc, i just went through this and phoz was super helpful in the process.
I was able to break the fill bolt loose using an open-end wrench up through the very-narrow gap underneath the fill bolt (u probably know, you have only 2 options to get a wrench on this fill bolt: from underneath it in that narrow gap between the t-case and the t-case support, or by coming in from the rear of the fill bolt (rear meaning toward the rear of the car next to the exhaust and heat shield). However, even though I could loosen the fill bolt from underneath (because I happened to be able to get the open-end wrench onto the hex bit), I had to tighten it using the "rear" opening because from underneath the wrench just didn't line up with the hex bit. I know exactly what you mean about not having any room to "move" the wrench. As I laid under the car, almost tearing-up as bossimo said (lol!), I thought "what I need is a short, stubby wrench". On a shot-in-the-dark, I went to Home Depot and there it was.....a set of "stubby" metric wrenches. BINGO! The stubby wrench allowed me to tighten the fill bolt enough using the "rear entrance" that I could then get my normal-length open-end wrench onto the fill bolt from underneath and crank it tight. As you know, it's all about being able to get the wrench on the bolt and being able to have enough room to move the wrench to turn the fill bolt to loosen (or tighten) it.
So, I would recommend trying the Husky "stubby" wrench set from Home Depot, it worked for me. They had 2 types, one was a "bag" of short wrenches, and one was a kit of stubby wrenches that were the Husky brand in a plastic case, not a bag.