OK, well I think that now I've walked this way, it's only fair that I answer my own question...
Yes, this is a nice variant and well worth doing, especially if you have stayed in Besain the night before as it's only 17km from there to Zegama which is a pretty short day for most people.
After leaving Segura you go down to the road and there's a red/white marker for a path going directly across country and up to Zerain. Zerain itself is charming, with an old building housing the shop, tourist info, sello, local crafts and products and books etc. Underneath is the bar/restaurant which doubles as the canteen for the village school. Zerain has its own website
http://www.zerain.com/index.php?lang=en (!). It also has its own hostel/albergue adjacent to the shop and school. Originally, we'd booked to stay there, but realised it would add 6km onto the hardest stage of the camino so we switched to Zegama instead.
The route out of Zerain follows the GR34 (the traditional route of the Transhumance) and the start is clearly marked.
https://www.flickr.com/photos/peregrino_tom/20127489550/in/album-72157654492550364/lightbox/
The rest of it is marked sufficiently, but economically, with one or two yellow arrows thrown in for good measure.
https://www.flickr.com/photos/peregrino_tom/20127454510/in/album-72157654492550364/lightbox/
The route is very nice, as far as it goes.
https://www.flickr.com/photos/peregrino_tom/20307099472/in/album-72157654492550364/lightbox/
After about 3km of the variant, the path comes to a house and onto a road and the GR34 turns right. At this point there are no markings for the camino and you should take the road to the left and snake down the hill, or else there's an off-road forestry path slightly to the right (that we were told is a short cut but we didn't take it, but we saw where it came out).
cheers, tom