Shel: one thing to also remember – you're not going to be able to use a virtual page view or an event to signal whether a plugin is installed or not (although you could modify the PV name) because that will create another server call and nullify bounce (you will get either a page view + event or a double page view, which are both considered non-bounces by analytics). A lot of people forget this and then they end up with super-low bounce rates (or zero!). A custom variable might be the best solution for sure.

Good luck! Have you posted this question at the GA forums for help, too?

http://www.google.com/support/forum/p/Google+Analytics/?hl=en

You could also explore features like Custom Variables in Google Analytics (http://goo.gl/NjLV) to see if there is a clever way for you to collect the data that someone was missing a plugin. Work with a GAAC if you need help, a list is here: http://www.bit.ly/gaac

If the download link is yours, it is pretty easy to encode that and capture the clicks on it with any web analytics tool.

-Avinash.