How many torques are on that?
Ideas:
- Weld a nut onto the top of it. Should be able to accomplish this with a MIG and a steady hand
- Cut a slot in the top with a rotary tool cutting wheel, use a large manual impact driver to crack it, or a dull chisel/flat punch and a hammer to tap it around (clearance might be an issue here)
- Rotary tool again, cut faces on two sides, locking pliers, turn them with channel locks
- Rotay tool yet again, cut faces on six sides, hammer a possibly sacrificial six point socket on it
Apply some heat to the bolt with a tiny tiny torch to hopefully not damage the aluminum nearby? Should help release the threadlocker.