Those cables do exist (e.g. http://www.datapro.net/products/usb-2-0 ... cable.html ), but you can't simulate keyboard and/or mouse using them afaik.
Why don't you want to use rdesktop via network? It does all you're asking for.
So, cut the power cable to the screen, and feed the 5V to this cable. It will feed the display and the pi.Mikumiku747 wrote: Thanks for all the great responses, but to those who suggested the 5v rail, I'm already using that as a power source for my screen.