Do any cars have storage capacity under the passengers seats?
If yes, is it comfortable?
If no, is it feasible?
Yes, some vehicles have storage under the passenger seats. I believe the best example of this are the Chrysler vans which has what they call "Stow and Go". The storage spaces there serve a dual purpose. They can either store cargo or they can store the seats themselves. This provides a lot of flexibility. If you need to haul something long, you can put all of the seats down and have a large flat surface to deal with. It works really well. Is it comfortable? That really depends on what you like, but the van I rented with stow and go worked very well and hauled a lot gear as well as my whole family.
To my knowledge, the Chrysler is the best example of storage space under seats, but I'm sure there are other examples as well.