I'd guess you have to bring you own seat cushion, because it do not think any will fit between the sliding mechanism. Also seat pitch will have to go up due to the aft placement of middle seat. I am also not sure what the sliding mechanism adds in weight or if it will pass the crash tests.
If you want to go this way then fold up seats are probably more practical, also because then both the window and the middle seat can be seated whilst the aisle seat is folded up, instead of only the window seat with this system.