Using a centerpunch and a short, stiff drill- like an 1/8" spotting drill- "chain drill" through one of the flats. In other words, drill a series of holes as close together as possible, parallel to the bolt.
Use a depth stop, a short drill, a piece of wood, whatever, to hold the depth of each hole to just a hair short of the thread peak.
Back up the nut with a wedge, an appropriately-sized scrap of barstock, or whatever- then smack the nut on the drill line with a sharp cold chisel.
All you have to do is crack it. That'll take the tension off, and you should be able to twist it off fairly easily with a normal wrench.
If you have a lot to do, then I suggest buying about a thousand double-end 1/8" stub-length drills, task half a dozen guys as drillers, a couple as chislers, and three or four as wrenchers.
Might not be the best system, and it'll depend on the skill of the drillers to not go too deep and damage the studs, but it'll work.
