[R-gui] tkStartGUI and on.exit

I'm using a modified version of tkStartGUI that creates a window with a
menubar.  My question is, once I've gotten the app up and running, how do I
get out?  tkStartGUI does something like this:

tkadd(quitMenu, "command", label = "Quit R", command = quote(q()))

but this does not run any of the on.exit() expressions. I use on.exit()
expressions to save stuff before quitting my application.  Is there a better
way?

Jeff