Sorry to resurrect a Zombie thread, but this issue doesn’t go away and I have similar issues.
For now, my 5 year old rides in the center, with her seat installed using the seat belt (no lower anchors in the center) and top tether. For the outboard seats, I replaced the inadequate head rests with a pair of front seat head rests. They don’t go down all the way, but that’s ok. This works for my 5’ tall 10 year old. I also have a couple of adult kids who ride with us regularly. The tallest of those is 6 feet tall and the larger head rest barely comes to the tops of his ears.
Eventually, I won’t be able to put a car seat or highback booster in the center anymore, and I’m interested in whether anybody has successfully installed an actual head restraint in that position, and what exactly was involved.
The picture is pretty fuzzy because we are driving away from the sun, but I thought it might be helpful.