tedster - 9:22 pm on Aug 10, 2012 (gmt 0)

It is a little worrying that the only DMCAs against sites I run have been against URLs that don't and have never existed... how much weighting will this be given?

Right now, I would let go of that worry. In this blog article Google did say they will be using VALID complaints. I'm sure there will be some people who poke at the edges of this to see what they can accomplish, but I doubt that their efforts will look "valid".