I would say that with placeholder maps especially then there is no need to get any permission - just start mapping. I could help out with height map and the original place holder raster but I wouldn't think anyone needs to ask my permission to map anything. Even in my Thrubmorton region id rather people just map it than not map because they thought they would be stepping on peoples toes. All that I would hope for is that new maps fit in with the surrounding stuff otherwise the CWBP would be just a bucket of disparate maps. There is so much uncharted territory that I cant think of a reason why anyone would want to start new areas. I think the best thing is to pick a region for which your artistic style matches so it looks a little better fit but even then I would not hold it against anyone if they went and did a wildly different style map.

If your just thinking of mapping a town, village, city, church, castle, inn, building, docks or whatever for no reason than the hell of it and you dont mind the CC license then look for something similar in the CWBP and map it for that and submit it for indexing into the wiki.