Problem solved! No modification necessary.
I have the same problem of my rear becoming numb. I had to add an icomfort air cushion to replace the stock gel pads on the Hobie Mirage kayaks. With the air cushion, I could go all day.
I didn't want to modify the Vantage seat by building a pillow case contraption with the air cushion and then try to anchor the contraption to the seat so I thought about just sitting with the cushion laying on top, but then if I ever dump the kayak, I would have to chase the cushion floating downstream.
I discovered that Hobie, knowingly or unknowingly, took care of this for us. Please see the following youtube video: http://www.youtube.com/watch?v=Dmf5HEm6RII
No modification to the seat. Buy a Thermarest lite self-inflating air cushion and slip it in under the mesh seating surface. On the very bottom, there is tight vinyl which essentially causes a hard seating surface. Problem solved! I bought the Thermarest lite on Amazon for $27. It's 15in wide. Before you purchase something else, make sure it'll fit in the pocket under the seat, also make sure the material covering the cushion will dry fast. I have a 2014 Pro Angler, and I have no idea if this mod for the Vantage seat will work for years prior or years after.