[Dukedesmo]

If it's a standard 'hoop' swingarm then, you should be able to get access for a narrow sawblade from underneath, maybe an air powered reciprocating type would be best?

Of course that entails lying on the floor, cutting upwards or lifting the bike, lying it down on it's side or better yet, removing the swingarm to get it on the bench.

But I still wonder if welding in a long bit into the head of the bolt might work? you'd have something to work on and the heat might help free it...