I don't see what the problem would be but you should let Challenge Control know what you are doing. You would probably have to let them know by phone as you leave Braemar (and after checking the weather forecast). Braemar to the Sheilin via Lochnaghar is a long day!

But you have the alternative of dropping down to Glas Allt Shiel for the night and Tarfside is an easy stroll the next day.

I have always thought that a Challenge form organised with Fair Weather Alternatives would make more sense than the current approach.