Finally got it. If anyone else finds themself struggling with this, you need a 10mm hex key to turn it. In my case, it took the hex key, an 18 inch piece of pipe and every bit of force I could throw at it with my 190lb body. When it broke free I honestly expected to look down and see the hex key snapped in two.
If you can get a 10mm hex socket on it with a breaker bar, that would probably be even better, but I simply couldn't wedge anything more than the hex key in there (tight quarters).
By far the toughest bolt in this entire project. Glad that's over