Interesting idea... there's no official way to do it, but if you create a transparent image in Photoshop at your screen's max resolution, you should be able to set that as the pic.
1. true... actually you might have to make it bigger if the folder scrolls more than the height or width of the screen.
2. no idea, but it can be done, so I'm told.
Great Idea, but it can't be done with the trasnparent image idea. OSX just takes the image on top of the white background...so, it just stays white...oh well. Good idea though.
You might be able to find a hack somewhere...but use at your own risk.
Everything in OSX has a transparency level and a mask, if you could only hack around inside the files like themers do, you might be able to find out where that particlar part is, so there is a decent chance it is possible.