No experience with your problem.
Projection screens usually have a retro-reflective coating similar to street signs such as "STOP" signs. These screens usually have little glass beads that send light approximately back in the direction that it came from with controlled spread. You can examine how they reflect light by punching a hole in a piece of opaque cardboard and shining a laser pointer through the hole. If the screen is brighter over, say, a cone of 20° then see if you can position the cameras outside this brighter reflection cone, probably a forward row and off to the side. You should also be able to determine this brightness by walking around when the projector is working. (Similar directionality for computer monitors and TVs.)
Rear Projection Screens. These also have directional characteristics similar to the retro reflective screens. These screens may spread a beam into a cone that is also intended to reach the seats of the conference room and not send light to the walls or ceiling, etc. .
A second more cumbersome method would be to place a neutral density filter some distance from the camera, 12"?, so that it appears over only the screen but not the speaker. It would work best if cut to the approximate width/height ratio of the screen. [Kodak Wratten plastic ND filters had good optical quality and could be cut to size.]