More recent evidence is that they will pull rooms to the waitlist PRIOR to putting them up on the website. I have some pretty strong evidence of this based on some waitlist rooms I have gotten recently.
However,
@crisi point here in the second paragraph is excellent. A waitlist can only be filled if EXACTLY what you ask for becomes available, AND no one has waitlists the same thing or some of it before you. So a lot of times its easier to piece together a stay by stalking the website and grabbing up days as they become available. I personally cannot be bothered to do this, but for some it is worth it.
I honestly have had every waitlist come through for 1-2 nights - maybe around a dozen that I've done. I have rarely had a waitlist come through that is for more than 4 nights. So I always think the key to wait listing is short windows. Better to request two 3-night waitlists than one 6-night one.