@Marcel: I thought about moving the print as well. Both methods have their challenges. Ultimately, moving an A0 foldout attached to a thick A2 book with binding falling apart would require a very sturdy platform to avoid any changes in the position of the foldout. I think moving the camera will be simpler for our purposes.
With the Canon 5D we'd be (hopefully) looking at imaging A4 sections of the print. At the angle used in this test I think you're right. The depth of field was pretty good but there was a bit of drop off in sharpness across the image. We have wired releases for our camera, and for this test I was using the self timer to avoid camera shake. Getting rails built shouldn't be too much of a problem but I will have to make a case for it and it will take a bit of time. A counter-weighted pole might be a better interim method for positioning the camera while we sort out the exact specifications for a set of rails.
Yes, you would need some kind of platform with wheels and put the print on top. With an A0 sized print that is not very practical.
Rails are not crazy expensive, especially if you buy the budget version (you don't need the kind of tolerances that are needed for CNC machines, as long as it slides smoothly it's fine). Here is a link to supported round rails (it's a webshop where I buy my Bosch profiles, just to give an indication of the prices):
stappenmotor.nl/Steppermotor/linear%20guide%20ways/supported%20linear%20ball%20bearing%20for%20cnc/supported%20linear%20guideway%20for%20cnc.htm
Do you plan to automate it, or move the camera by hand? If you want to cover an A0 print with A4 sized shots then that is quite a lot of photos (including overlap).
I would love to have a fully automated XY rig myself, also to be able to do reproduceable tests with Photoscan.