Pretty common steganographic technique, really.
Additionally, if I copy-paste text like this are the invisible characters preserved? Are there a bunch of extra spaces somewhere?
IMHO it isn't a feasible way of watermarking text though - as someone would promptly come up with a website that undid such substitutions.
It doesn't matter since there's no one-pass solution to counterfeiting.
You have the right of it-- the best you can hope for is adding more complexity to the product, which adds steps to their workflow and increases the chances of the counterfeiter overlooking any particular detail that you know to look for.
