I have been playing with this for a while, but the closest I have gotten is a button that opens the "Paste Special" dialog box and requires another couple of mouse clicks to paste the contents of the clipboard as unformatted text.
So often I am doing a copy-paste from a web site into a document where I don't want the additional baggage of the HTML formatting, it would be nice to be able to do this with a shortcut key or a toolbar button.