You need to be able to provide independant pressure to the bolt in the direction you need it to move... which the threads usually do...
You can JB Weld another bolt into that Allen leaving some distance between the top of the Allen and the bottom of the bolt you JBWeld in....and make a metal puller from wire to apply the pressure with while you turn the bolt... or if you can get to the other side and apply even a little pressure it should move to a place where the threads will again apply the movement.