Yeah, this is difficult for me to explain!
It's not configured this way now because of the imagebuttons, but here is what I had:
The thumbnailscroller was set to autohide with a height configured large enough to almost touch the top of an image. The controls were set to remain visible as you said. When moving the mouse down toward to the thumbnailscroller it would activate and cover the maincontrols, which is the feature I wanted, although not cover the imagebuttons.
I just recreated this on my site for you to see what this crazy guy is talking about! It's crude as I didn't worry about making opacity match, etc, but I think it will make more sense visually then textually. Do you still have my site information I sent via PM a few days ago?