[JKT]

FWIW, although it doesn't exactly do what you are asking, the blocks plugin for Rapidweaver from www.yourhead.com goes part of the way there. Perhaps drop the developer a line to see if he could code what you are looking for.

Edit: Naturally, the big bear in the room is IE6 which would screw up any WYSIWYG design that was done to web standards. The app would have to explicitly produce the screwy code need to make it IE6 compatible which may not make it something trivial to develop.