Not that this is pretty but the jack should be able to slip between the seat in the middle. This is how I have my shovel mounted (with the long handle going up between the seats.
If you wanted (way to much work
) you could make an arm rest for the back and then use the space below for the jack. You could cut a small square in the seats so the the stuff would fit thru easy. Kinda like the ski bags for cars (things that come from the truck) except the arm rest would be made of hard materail and have a proper finish on top.
Yea I know I have to much time on my hands to think this stuff up.