Re: Banana Seat on a Mountain bike.
Some of the 20" Schwinn Krates had little shock absorbers in the sissy bar.
You could fab something with elastomers, springs or both for your sissy bar.
The lower legs of a suspension fork might be added to the ends of the sissy bar.
I have never sat on a comfortable banana seat. Maybe one of the fabric covered bananas like the lowriders use may help.
“You're only young once, but you can be immature as long as you like" -Unknown