I think i have seen something like this posted before.
It's a good idea but there could be some problems..
First of all, as Meakel said, false awakenings aren't always perfect and the sign might not appear in your dream. In a dream we are on the whole less aware of our surroundings and so the fact that the sign is missing will go unnoticed.
Also, if it's a false awakening in the middle of the night....isn't it going to be too dark to read the sign, especially if it's across the room from you on the wall?
Just a couple of things i thought of - but on the whole it sounds like a good plan