This has been noted by manufacturers and some now are making bob-tailed TT helmets that have slightly more drag than the long-tailed variety in optimal position, but less overall drag as the rider moves his head around as they do in real world racing. Kask was among the first and their bob-tailed TT helmets are often seen on Team Sky riders. Bell/Giro current TT helmets have short tails compared to the earlier offerings. Louis Garneau also makes a short tailed TT helmet, the Vorttice.
Hinged? Not sure, but probably not UCI kosher.