Your XUL apps approach sounds interesting. Can you offer any pointers on what you do and how others could do it too?
Basically you open the XUL app for the service you want to access. It acts as a chromeless firefox browser and intercepts any links you click. If it's an external link it opens it in your default browser where you wouldn't be logged into google preserving a bit of anonymity.
I also found this link http://www.ibm.com/developerworks/xml/tutorials/x-xulintro/ after a little searching. I'll try it out.