Hey Ian-
check this thread:
Screen Printing Posters
there's a little in there. i haven't seen too much on poster printing around here, and i'd like to play around with it. from what i understand you will need to look at a table-top press/ clamp setup and fix some sort of registration jig. this way you can register each poster, and print a run of all one color, filling your drying rack as you go. after they are dry, register the new screen and do your next run. etcetera. you will not be using textile inks, of course, and i think there's some info in the above posted thread on that too. post back when you learn tons on this as i am also curious.
derek