The third row is perfectly safe for a properly restrained child. Rear impacts are typically much less common and much less severe than side impacts, for example, and a light child combined with a good solid volvo seat is a much better combination than a heavy adult in a cheaper make's seat. The third row danger data comes from lots of older cars with only lapbelts, no head support, and no seatbelt laws...your kiddo is considerably safer in the third row of a volvo, buckled properly, with head support than he would ever be in a 1984 Caravan with no seatbelt or headrests.
But anyway, if you were to modify your car in anyway, it would be best to get approval direct from Volvo for it (VW, for example, frowns upon adding anything to a car to make it safer...such as foam on the doors and head panels...).
Anyway, better to ask than not, so it's good you did! It couldn't hurt to call Volvo and at least ask them what sort of testing it does...I believe the passenger compartment does include all seating positions, so the third row isn't actually a crumple zone or anything, and should be reasonably protective.
Good luck!