I wonder how much of the head injury problem is caused by the car-style seatbelts and cr@ppy seats in the PA28/C172 style aeroplanes? Excluding aerobatics, as IO-540 says a bonedome is only going to be any use in a low speed accident - which almost by definition is one during take-off or landing (forced or otherwise). Wouldn't it be better to make sure that the seat and harness stop your head from getting anywhere near something solid?
The seat rails in the PA28s I used to fly seemed very flimsy (and by reputation the Cessna ones are worse), and I would be surprised if they stopped the seat sliding forward if you hit something solid. Likewise, the three-point seatbelt is total rubbish - there was one poor sod in the AAIB reports recently who flew a perfect forced landing after engine failure only to die from head injuries thanks to the seatbelt being useless when he hit something after landing. I've even flown an aeroplane (once) with just a lap-strap
