We have a beta version (still in development) of Boxcar 2 Growl notification support.

You can download beta plugin from this link: Boxcar2.growlView.zip.

To configure it, open Growl preferences.
Go to Displays, then Boxcar 2 Action and add your Boxcar2 access token
You may add Boxcar 2 in default actions list or use it at application level