My friend and colleague Keith Casey pointed out to me that Thunderbird has a Provider for Google Calendar add-on that works with Lightning, which I already had installed. So I install the GCal add-on and restarted Thunderbird. No visible change. I checked the Add-ons area only to find that the entry for the GCal add-on was displaying the message "Requires additional items" with no way to see what said items were. A search prompted me to check the version of the Lightning add-on, which I found to be 0.8 because I’d already installed the Ubuntu package for it and that was the latest version available.

As it turns out, the Ubuntu wiki has instructions for what I figured out on my own the first time around: version 0.9 of the Lightning extension has to be downloaded and installed manually for 64bit builds. After that, the GCal add-on was right as rain. Adding a calendar was as simply as accessing Google Calendar via the web, going to Calendar Settings for an individual calendar, and using the XML link for that calendar when adding the calendar to Lightning using new new Google Calendar option added by the GCal add-on.