I don't have the SLS-2 bundle so it's difficult for me to give concrete instructions. You are right, the original DAVID setup with projector and camera mounted on a common rail cannot be used for this purpose. And the specs for SLS-2 are made for objects >= 60mm.
So what to do? I would mount the camera directly above the projector.
Or, give your idea of a zoom or tele lens a try. f should be circa 16 or 20mm. I am sure it will work. But don't buy a DC automatic iris lens which you linked to 2 days ago.
Edit: I have just made a test with my modified Logitech C600 and a cheap 16 mm CS-mount tele lens:http://www.pollin.de/shop/t/MTE2OTA5OTk ... ktive.html
I put a 10 Cent coin between the thread. The distance from frontside of the lens to a 1€ coin as an object was 115mm. The image of the 1€ coin was format-filling (in vertical direction).
Maybe a combination of using a tele lens with extension adapter and mounting the camera above the projector is the best.