Airplane Seat Cushion for almost 5 yo?

In September we're headed DEN-LIH with our 3 yo and 4.75 yo (layover in LAX). The 3yo will ride in style (ha!) in a Cosco Scenera but I'm struggling with options for the older one.

In a random turn of events, I'm actually on a different, earlier flight than DH and the kids, so I thought I could lighten his load by taking the 5 yo's carseat (Evenflo Securekid 400) with me and gate checking it.

We had the CARES harness but DH hated installing it, I hated how he slid down in the seat, and he hated the whole darn thing.

My biggest concern with using nothing but the seatbelt is that he can't see out the window, so I am considering getting some kind of booster cushion for him. He's about 36 lbs, 37" or so.

My understanding is that I can't just use a regular backless car booster since they're not FAA approved, so what about something like an inflatable camp seat http://www.rei.com/product/845302/rei-sit-pad or an inflatable cushion geared toward travelers http://www.magellans.com/magellan-s-luxe-inflatable-seat-cushion ?

Then my question is, how is the Magellan's seat cushion any different from a Bubble Bum? Those state they can't be used on a plane (not FAA approved) but aren't they, in essence, just another inflatable cushion?

Anyone have any experience with either using a backless booster (we have the Graco SafeSecure Turbobooster in Grandma's car that converts to a low booster) on a plane (sneakily) or using some alternative seat cushion?

If it comes down to it DH will just take both car seats and be fine, but I already feel guilty that I get to enjoy a nice long kid-free flight (that ends in Hawaii, no less).

This thread has information on why a booster seat isn't approved for airplane use: http://www.car-seat.org/showthread.php?t=168758
but it does mention one can be used during the "cruise" portion of the flight when the seatbelt sign is off so a child can see out the window.

Personally, I would still send the car seat with the child though as the seat could still be subject to mistreatment, damage, or loss when it is gate-checked.

Thanks for the reply! I realized after I posted that he'll need the carseat to get from our house to the airport, so it's a moot point now, but I'll probably need the info again in the future!