I can say with certainty that Kevin's work is not recast *at all* and is indeed a 100% fan sculpt. I'm sure he'll take it as a compliment that you think it's recast, but you can go waaaay back to his v1 sculpts which he posted on the Legion boards to see that it's 100% recast free.
The reason it's not allowed for Centurion is that while the helmet looks great from a side shot, it looks a bit off when viewed direclty from the front. This is the same reason why MRCE is not allowed for Centurion - looks fine from the side but not quite right from the front. RT & TM are not 100% screen derived either, yet both work just fine for Centurion.