OK finally an area were I can add some valuable input. I am a Firefighter/Paramedic and have been in charge of the Child Safety Seat Program for 12 years now.
If you are installing a forward facing seat in a GN, using the car seat belt (to hold in the seat) then you do not have to use the top tether strap located on the car seat. Disclaimer!! car seats change all the time so the only way to tell for sure is to read the car seat instruction manual. I know it's boring and usually confusing but that is where the necessary install info. is. The top tether and lower latch system were meant to be used together, only. A car seat belt holding the car seat in does not use the tether or latch points at all (stand alone system).
One expectation that I make for my own safety is: even when using the car seat as a booster (no 5-point harness installed) I will still loosely clip the top tether to the deck clip. Just in case I am in an accident and DO NOT have my kids with me, I do not want that seat to come flying forward and knock me out.
Yes all auto manufactures have to install a top tether clip is asked to, however, have you ever seen the rear deck on a GN. It is very thin and would not hold up to the crash forces generated in a front end collision. So IMO you would be wasting your time having them throw one in. Using the middle rear seat position, and the car seat belt, is your easiest and best bet if the child seat has a 5-point harness. Booster seats must be placed on the outboard positions because of the added shoulder belt protection.
Your technically not allowed to put towels under the seat, reason being the towels will compress over time and the car seat should be tightened to less then 1" of play forwards and backwards. So this set up would fail under a short amount of time. Solution: you are allowed to use the 1/4" rubber mats that are sold in most stores that carry car seats. These rubber mats are threefold, they stop car seat from sliding around, prevent stains from spilled drinks, and lastly they protect the oem seats from damage from the bottom of that child safety seat.
Again, please double check your car seat instruction manual. Hope this helps you guys (and little ones) out.
Busyman