Using DisplayPort hubs, splitting the massive meta-screen
The Matrox TripleHead2Go includes their Powerdesk driver that can apparently send windows and the dock to specific displays. I'm not sure how far the illusion goes that they are indeed separate displays instead of the one logical display they represent.

Have you actually tried USB monitors? If used for coding and/or docs, I don't think their theoretical lack in bandwidth creates much of a practical problem. If you plan on gaming or watching videos on them, that's another matter. I wouldn't rule them out for ordinary work-type jobs without checking them out myself first.