In a permanent setting you can file the projector's aperture plate at an angle so that it projects straight vertically. This is more common, of course, with 35mm projection but the same principle would apply to 16mm.
From: Tom B Whiteside <(address suppressed)>
Optical solutions to keystone problems are common in video projectors; in film the only solution that I know is to change the angle of the screen. If you are always projecting from the same angle (ie, from the booth, not from the floor of the room) this can be a permanent solution.
For info on FrameWorks, contact Pip Chodorov at <(address suppressed)>.