To add more context to this, adding planned/proposed features to OSM is a bit controversial because:
a) those features don't represent current state
b) plans are often cancelled but those features typically stay in OSM for a long time until someone notices that they should be removed.

But anyway since you say that it will be done within a year and you are directly involved in the project, I personally don't see a problem having the trail mapped (tagged as proposed/planned).