I believe that it's fixed onto the bracing ring between the body and the bell and then slanting down the side of the body, presumably being fixed on the outside of the bow tube.
It's a bit like this peg that I spotted being used by the bari player with Amy Winehouse at Glastonbury a couple of years ago. I think he is called Jim Corry
http://www.jimcorry.co.uk/ and he may have the bari spike from the same source as Joel Purnell for his tenor spike, as they seem to come from the same part of North England (Yorkshire).
Not very clear from this screen capture, but the bari one seems to have a telescopic peg with the tube angling down from a top fixing at the back of the bracing ring.
Rhys