So I made a fighter with 17 CON (after a respec), then got the hair and made it 18.
Leveling up from 5 to 6 I went from 54 to 64, so 6 + CON bonus seems to work, but I noticed on the actual level up screen I went from 49 to 58, which means it wasn't considering the hair on that screen. It also showed my base stats on creation, er, respec, meaning 17 for CON, so that also works out.
It just seems a bit odd to not consider the hair bonus as part of your basic stats. In D&D, actually boosting a stat was a rare thing indeed precisely because it did alter everything, as opposed to a rare piece of equipment, which could just as easily be removed.