Sounds like a good idea. It would definitely allow for some nice effects, especially on touchscreen targets.
Which brings my to a point I already raised in the custom status bar thread: It would be great if menus had a (invisible) back-button, either set by the sbs or the menu itself.
Imagine you'd bring up the sound settings in a small popup window drawn on top of the WPS. Now to go back to the WPS you'd have to go back all the way to the main menu just to get back to the WPS while you could see it in the background of the window all the time.
That'd be pretty awkward and outright ridiculous on touchscreen devices where you'd think a single press on the WPS would bring you back. This is already annoying and would get even worse once menus can be transformed into WPS/SBS popups^^
To sum up: I really really like your idea. Just think about usability as well

EDIT: Honestly, If I understand everything, you could make really awesome things out of this. I hope this gets implemented