If you want a seatbelt only booster with 'sleep slump' support, Recaro has a high back, head support booster seat with a weight limit of 120+ lbs. I got one for my seven year old in midnight (it's a soft micro fiber versus a poly blend I didn't like).
I got it because of the exact reason you are stating.....the sleeper's slump! And in our case, the 'DS in the car gamer's' slump!
Check out www.recarochildseat.com for more info. They have a one dollar shipping insurance policy...you pay one dollar and if the seat doesn't work for you, you can send it back for free.
P.S. My son says it's super comfortable to sit in too.
Also....the recaro prosport is a harness to seatbelt booster. Harness up to a crazy weight (my gf's seven year old chunk *sorry but it's true*) is still harnessed on long trips and it converts to a seat belt booster, at the earliest 40 lbs, but as late as whenever the harness doesn't fit anymore, and can be used as a seat belt booster up until 120 lbs. Anyway, that's all on line as well......
I, just yesterday, bought one recaro prosport and one britax frontier 85. I also have the reacaro probooster seat. The one thing I don't like about recaro, and it's really petty because I think for safety reacaro can't be beat, is the fact that recaro doesn't have cup holders and my kids have been spoiled with the ones they have now. Thus, the Britax (as long as all 3 car seats fit in my back seat because the britax is 19.5 wide instead of the 18 inches of recaro).
Believe you me...I've researched!!!