MT, have you given any thought to spring mounting the rear of the seat? If you have welding capabilities fabricating mounts shouldn't be a problem for you. As I've posted in another 'seat' thread, I've used automotive valve springs with good results. I've also used the springs and spring perches cut from a standard bicycle saddle.
It's been my experience that the front mount doesn't need to swivel but instead the seat will absorb the spring action enough that it can be mounted as designed to the seat post. If you make the springs adjustable you can 'fine tune' the seat to your weight and riding style.
Also check your local foam supplier and ask for 'motorcycle foam' or the most dense foam they can get for you. Foam sold for softer applications such as sofa cushions generally isn't firm enough for a bicycle/motorcycle seat. Memory foam is an option too but again, it needs to be firm to absorb road shock and a non-suspended rear wheel.
Keep us informed on this project. Everyone would like a more comfy ride and it will be interesting to see what you come up with.