Richard if think your have put your finger on the flaw in the Rambler's basic strategy. So far I have only had a look at the website, tried to sign in but the identity I used for their other footpath scheme seems to have lost my details.
The flaw is the different purposes contributors may be accessing the map for. By blocking a square and not allowing the contribution to be visible it is impossible to assess the continuity of way. Continuity of Way is essential to assess the relevance of away for current and future use, it brings into play other factors that give circumstantial insight into why a way may have been left off the Definitive Map. Much of this can be found out by going out into the countryside and looking at the terrain and speaking to old genuine country people. As I have not yet bothered with this too much yet I do not know if there is a place to leave comments after completing square, as I do not wish to compromise a square.
My suspicions are aroused because I have just looked at an are I have researched; three squares are greyed out and they are the obvious ones but the square that really provides interest to the overall way is available. It would require a bit of inside knowledge to know this. Are the greyed out squares blocked because they have been genuinely filled in, have they been blocked by the landowner or even the county council because they know there is an embarrassing issue here.
However is there is genuine interest in the continuity of way there is always the Library of Scotland's maps to check out. The 1:50k maps are not the best for this purpose, the seamless 1:25k scale is better for spotting anomalies. Then comparing 1880, 1901 and the two pre 1949 editions are better. Further details are sometime found on the 1 mile/6in scale.
Ninethace, when I first started taking an interest I learn that Devon was one of the worst counties for interpreting the DM, I think that was on the old ramblers forum.
The other titbit of interest is Devon is the home of Sarah Slade, adviser on Access for the CLA and according to a fellow member of the Stepping Forward Initiative a rabid anti Access.