I have'nt cycled the Route 71 from Richmond and Northallerton, but I will of cycled most of the route, pre-Sustrans.
I don't think the map is out as yet and seems to be a on-going project, but concerning to my Sustrans Supporter NCN map 2008. This clearly show the NCN 71 being fully open from Whitehaven to Northallerton.
I suppose there is nothing stopping you from doing the ride, just a bit of guess-work.
This link will help
http://www.rangernetnyorks.btik.com/gallery/789814475.ikml
http://www.rangernetnyorks.btik.com/news/789819369.ikml
It's funny, that the route does not go through Richmond at all.
However I will be attending the Ranger's Day at Durham in a week's time, so should hopefully get some inside information from the people who know the in's and out.