Shortest kid in the middle ! kidding sorry :P But in all seriousness...
Quick and reversible solutions might be to use some of those special cushion supports for people with neck/back issues that need extra support when sitting.. Setting one of those up might help... Or like the cushions for people travelling, may want to attatch them with velcro strips or 3M tape or something so they don't move around like you said in case of an accident. However.... I don't know they'd hold up that way in an accident.
It would certainly help with comfort, not sure about safety..
If you want to get into spending more money I am sure a lot of custom fabrication shops could easily go right in and install an extra head-rest. More costly, but perhaps a better bet if safety is the primary concern. Your dealer may have connections to help you out with that, but often going through a dealer just ends up costing you more.
The cushion idea could be a good temporary fix until you figure out something more secure though.
I am assuming the primary reason for not having headrests in the middle seat would be due to visibility in the vehicle? Which might be tied into actual legal factors... I wonder if having this visibility hindered by an aftermarket modification might in some places place more liability on someone who had willingly added a visibility obstruction in this way to their vehicle, in the event of an accident? Especially if the impact were to have anything to do with the rear of your truck. Might be worth looking into, or maybe someone here knows more..
I think an extra headrest would be a good idea personally. Nobody likes being stuck in the middle seat. Some kind of cushioning back there I would imagine would greatly reduce the chance of whiplash or injury to the head.