Hi,
our DD (age 6, classic Kanner's autism) does this, have mentioned it to the paed by email, but not due to see him for a while. We think it's partly sensory and possibly related to an external stress that DD can't describe... e.g. at the moment she's still settling into Year 2 with a new teacher who hasn't had an ASD child in the class before. We noticed the hair-eating thing too... definitely yeuch!... but I said to her very solemnly that "the hair would stay in her tummy forever", not like the cat (who brings up hairballs that she's seen) and that seemed to get the point across!
Interestingly, it's started at the same time as a whole load of other stuff: sleeping 'pattern' much, much worse than usual, upset tum when she goes to the loo, all of which make me think it's some external change triggered all the behaviours.
Sorry I haven't got any wonder cure, but we seem to have something that's fairly common.
Enrico