What kind of time frame do you have? Between asking here and if you have to order the glass..etc Might help to know what kind of time frame you are really talking about so that ideas anyone can think up will have that as part of the consideration.
What posters are this? How would it finally be kept after the shoot? Rolled up again? Or mounted?..etc.
The first thing that came to mind was a hand held photo scanner back in the old days.. I am wonder if they still sell that, if they do you can scan in by portions and then stich in photoshop.
The other way will be use a flat bed scanner like Otega mentioned. Take the lid off off the scanner and just do it segment by segment.. it should not be that hard compared to all the time it takes ti wait for more ideas, order the glass, setup..etc. How big a reproduction are you using the shot for final production work? Print out to full life size or bigger???