It took a lot of ferreting out, but I eventually learned that you can put a <version/> tag in your site.xml, and Maven will put the project version number into the generated site. By default, it appears at the top, next to the (default position of) the publish date. <version/> can have a position attribute, as with <publishDate/>.
And by ferreting, I mean about an hour of following broken links and scanning undocumented code, jumping to random places in the Maven SVN, trying experiments, and other forms of unnecessary guesswork. As usual, it would have been nice if it was documented; likewise it would be nice if the Maven brats ate their own fucking dog food when it comes to site generation.
As is, if you want to figure this stuff out, your have to guess and hunt around to figure out which of the bajillion plugins is responsible, then you have to guess where the site for that plugin is generated (or try a Google search and hunt around), and then guess where the source code is in the repository (since the documentation is usually fucking wrong and entirely missing). Fucking project comprehension my ass.
The Maven crew has always been about slopping out undocumented, broken shit, rather than actually producing useful tools. I can't wait to get off of it forever.
End rant. I still like the concept of the repository and the transitive dependencies; that aspect of Maven is worthwhile, but as a build tool, it sucks up far more time and energy than it saves. Possibly an order of magnitude more.