It may well be a backward thingy, I'm always suspicious of those... on the other hand I'm pretty sure it doesn't derive from any of the old usages of nonce either (although I too laugh every time I hear it in that sort of context). The only thing is that they do say "nonce wing" don't they and the word tends to be used to describe various types of untouchable people - sex offenders and child killers which I wouldn't instinctively group together except in prison so I wondered if the origins might lie there. I've thought about this enough now.