"Good enough" HDMI slide viewer/scanner
So far I have a form-factor, roughly the same as a pringles can, but square. Slide backlight at one end, slide slot just in front with a microswitch in the bottom connected to a GPIO pin. Unlike a normal slide viewer I'll just be taking a still image and projecting that, instead of video. That should get me access to the high resolution modes of the camera (>1080P). I did some experiments with my Canon 20D and a macro lens, which were... OK, but not great. My current alternative is a Sony 8MP USB webcam designed for fitting to a stereo microscope. They are about $70 on ebay. I'm still not totally clear on how to make it focus from a few inches away, but that's why it's a project, right? :-)
So the rest of the shape for the viewer is taken up with an optical-bench type arrangement, since I don't know yet how far away the camera will need to be from the slide surface to get a full-frame image. Wherever that is, the Pi will go directly behind it. I'll add a few buttons to the case to allow you to cycle through already-captured images, and possibly a second SD card accessed via SPI, so that the images can be removed without crashing the Pi.
At the end, the aim of the game is to be able to have "one last real slideshow" of all these slides, with a byproduct being a stack of SD cards of 8MP-ish images of them all.