Re: Added good springs to my saddle.
I also considered using the shock from a full suspension mountain bike. Either Wheelbender or Xseler (I can't remember which right now) has built one that way. And I liked the look of it.
But I think I've found an advantage over that design; my only mount point is the seat post itself. There's no need to mount to the frame below. This'll make seat height adjustment, or swapping seats for that matter, a lot simpler.