Solved
I couldn't leave it alone and solved my own problem.
I ended up using the grinder. I ground down to opposite points to as close to the shaft as I dared.
I just started the cut with the grinder, then finished the cut with a hacksaw.
I then just grabbed both ends with vice grips and "peeled" the nut open.
Fortunately, the threads on the shaft are fine!
In the following pic, I put the nut back on the shaft to take the picture. It was still solid all the way around after I made the cuts:
nut ground
Nut Removed