This Supreme Court decision only allows prisoners to grow a half inch beard instead of a quarter inch one allowed by Arkansas, at least according to the press, but half an inch appears to be all the inmate asked for.
http://www.newser.com/story/201552/supreme-court-unanimous-on-inmate-beard-length.html?utm_source=part&utm_medium=inbox&utm_campaign=newser
Not strictly about hair length per se, but of interest, I think.
Very interesting, Alun -- although I didn't read the article, I remember reading somewhere a few months ago about the fear of prisoners hiding weapons in their beards and/or hair; so I'd say this is definitely of interest.
- Ken