If you're using OTS2, you'll modify this diddy of the specification, but that's cool :)
I guess you have to use a theme that loads mzscript (so it's loaded last for the theme's startup scripts)

I suppose you could do something like:

That seems a bit tough to trigger though. I personally have a left double-click do !PopupTheme.

(I should post some scripting for generic mouse hold and double-clicking actions for jDesk but never get around to cleaning it up first)