Does anyone know of a method of intercepting the popup close event and cancelling it if need be?

I'd like to run a "has the form been modified" type check when the user clicks on the popup window's close button. Basically, I'd like to warn the user if he/she might be about to lose some work. Is this possible with the GreyBox javascript module? I've tried simply binding a handler to the GB_close element, which doesn't work. That's not too surprising, as GreyBox.jx has installed its own handler. I haven't found any mention of a hook that could be used in the GreyBox documentation, but am hoping that there still is a method of doing this.

Hans