Good call. Hit it again with fresh eyes and a night soaking in lube!
Well, the more recent go-to method for separating the two stuck halves is not going to be as easy with the bolt still in there, but if the drill bit is sharp enough, maybe the bolt won't impede things too much.
And that's to literally drill straight upward through the bottom. Ahh, but being the one behind the seat, that means it's straight on top of the frame, giving you zero room for any big tools. I see why you ruled out the Sawzall quickly.
But maybe the drill isn't such a bad idea here. Just go at it from the top instead.
Yes, it would probably take a week (or an hour, whichever comes first) or seem like it, to drill down through the bolt.
You're not trying to save threads or anything, so getting it perfectly centered is strictly to make it easier to drill. Nothing of the old stuff to save!
Anyway, that's my late-night take on it. Hopefully fresher minds will think of something.
In the meantime, keep banging away. Just harder and with a bigger hammer!
Paul