Thank you for your kind words!
I concur about the OHP lenses being of dubious quality, having tried using them in my youth to form primitive imaging systems. They did work, sort-of, but not well. I always thought it was pretty amazing that they managed to produce such readable images on a projection screen.
The usually-floppy page magnifiers are another case in point. They work well enough to give some magnification of high-contrast print, an application that is tolerant of misalignment and flexing, but is quite different from trying to form a well-focused image on (eg) a camera sensor.
One thing I have found so far is that the Fresnel lenses can be used, after a fashion, as a macro close-up lens with an existing camera focused at infinity (rather like the laser-type germanium lenses that get put on fixed-focus Flir etc cameras for close-up work). My limited magnifying experiments so far have used the Fresnels close to the object rather than close to the camera.
I think I'll take the tip of using a temperature-controlled soldering iron as a target, at least for the first wee while: not because of its temperature regulation, which isn't likely to be that accurate, but because I can dial it down to something like 150°C or lower and keep it there indefinitely. As others have noted, a soldering iron also provides a well-defined target so that one can check focus and resolution with ease.
Experiments continue; I hope soon to be in a position to post some sample images.