OK, so to answer my own question.
We need a wide image that covers all the homescreens, it isn't just repeated on each of them. The Xoom screen is a standard 1280 x 800 WXGA and has 5 home screens, so a wallpaper needs to be (5 x 1280) x 800. I've found that IrfanView is spectacularly good at this; I'd imagine most people already have it installed, and it has a tool under Image | Create Panorama that easily aligns single images then produces a single large image. The How-Tos I saw for software like Corel Paintshop were all using the method of 'paste it in as a selection, the align it by hand...'
Using IrfanView there could obviously be different images for each homescreen, too. It is so easy, and it's free. 10/10 IrfanView
Rob