Baby Boris is now fully functional and we gave him the first test run shake down. The radiant energy of the hugely powerful light source quickly heated up the test slide made on thin polyester film stock taped to a piece of window glass and started to buckle it! This was something I had not considered.
We then tried sandwiching an Estar based (Kodak trade name for the heavy polyester base they use for their sheet film) transparency between two pieces of glass with no mask. The combination of the thicker film along with the heavy glass on both sides allowed the slide to easily stay in the projector two minutes with no apparent damage and it was cool enough to handle as well. Boris stayed nice and cool at the slide stage; no problems there, although he does heat up the room.
So, the answer is the slides will need to be masked in-camera and then just trimmed into the mount which is a simple sandwich of two pieces of window glass cut to size and carefully taped on the edge. This is actually a more elegant solution and avoids a lot of gunky tape. Additionally, I may need to cycle between two projectors if I want to keep an image up a long length of time. The plot certainly thickens!
The photo is by Frank Siciliano of Steamed Punk and FPS Design. I was there while Frank was making this image. The working method was nothing less than impressive and the resulting image speaks for itself---amazing.