Peter
I doubt this line would have been viable you just need to look at the number of branch lines in the Northumberland ,Scottish borders that closed early or were axed by Beeching. Being rural passenger traffic would be minimal so you are looking at freight again not many towns in this area.One interesting thought the RAF base at Spadeadam would be a good source of traffic. Most lines in this area were single track with loops at stations .A line via Carlisle is a good option but again the area is very isolated .As they say it is your layout so why not.Good Luck.