opaque projector lens
I have one of those eBay 18" lenses on the way, too. I am really interested to see if these lenses are corrected for chromatic aberration. The easiest way to tell is to project an image of something with a black background and some white text around the edges. If the letters have multi-colored fringes, then it is not color-corrected. If they look sharp, then it is.
I think opaque projectors are pretty simple: They just throw as much light as possible on the object surface. That means a high-wattage lamp, fan, and reflectors. The diffuse light from the object is usually bounced off a front-surface mirror, so the object can lay horizontally in the tray. Then the light goes through a great big lens to get the image focussed on the screen. The lens has to have a large diameter, since the light from the object goes in all directions. Only a small part of that light gets to the lens, so the projected image is not very bright.
I think you could remove (or just ignore) the lamp & fan, open up the bottom so you could put an LCD exactly where the copy tray is, and then add a MH lamp, reflector, fan, and fresnels under the LCD. It would not be difficult to make a nice looking box for the stuff under the opaque projector. I think it could look very professional. The only problem might be that the tray area, mirror, and lens FOV might not be quite large enough for a 15" LCD. You might be limited to a 14" or smaller. Actually, an 8" Hami, or a 7" Lilliput would work fine, to give you a long-throw projector you could put at the back of the room.