Ben Northrup emailed me about his PMDReports project. To quote from his project page, "Whereas PMD generates and displays code quality statistics at a source code level, PMDReports persists and aggregates these statistics so that code quality can be viewed from a more macro, component level."
I got the latest version (0.8.2) and ran it on PMD's util package; here's the resulting "Component Quality Snapshot Today" report. You can drill down into the rule violations, and although I just ran it on one component I could see how this could be the makings of a nice dashboard. It would also be interesting to see those trend graphs after a few days.