Joe,
From experience, it would be a miracle if you could bang the seat pivot out. If it was a bolt, with the head twisted off, it's possibly threaded into the other side of the seat frame anyway. If you've tried already banging, I wouldn't bother again. But if you insist start dousing it in you favourite penetrant (eucalyptus oil is a left-field candidate, but google will give you 100 others) then use a torch (I use a small gas torch that burns MAP gas) to heat the lug before you try again.
That said, it won't work. Drill it out, or get someone to drill it out for you. Hitting things with big hammers will always lead to the "now why did I do that?" moment. Avoid.
Leon