It's now been 8 months since I posed the question, and I've spent a lot of time evaluating various answers here (as well as suggestions posed in other places). I'm confident saying that there was a "best" answer here based on an 8 month project. Indeed, on the basis of a vast amount of empirical data collected, there is a right answer here, which is Spiral's, and there are also some wrong answers.
I asked about the existence of a list of all commercially available fonts. Thank you,, Spiral, for pointing out Typedia and Identifont. Over the past 8 months I've programmatically encountered 3,753 different named fonts, found across > 5,200,000 web pages on the public Internet. As it happens, I was able to find all but 7 of these fonts listed on Typedia. These three fonts turned out to be created and owned by individuals who were not making them publicly available.
On the basis of this empirical data, I'm comfortable saying that, for all practical purposes, a comprehensive list of all commercially available fonts does in fact exist in the form of the crowd-sourced Typedia site. The idea that there are significant numbers of unlisted commercially available fonts floating around the Internet, which I myself had previously believed, is now urban legend. If a font designer wants to make a font commercially available, it stands to reason that he will take the small effort required to list it at no charge on sites like Typedia or Ideafont. And if that designer chooses not to, then another individual will see the opportunity to gain social capital by listing it for him.
This is not to say that there is not a vast array of special-purpose or limited edition fonts in existence, but only that these are not "commercially available".