I thought about trying a jeweller's magnifying loupe (the thing they use to look at gemstones), but they looked a bit heavy.
What I really need is some kind of curved mirror. After looking up in wikipedia, I reckon I needed a short section of concave curved mirror with a focal length of only a few cm. OK, where the hell do you get one of those?
Well, if you extend a curved mirror, you end up with a sphere. So, a mirrored sphere - a few cm across? Well, your local christmas shop!
To cut a small square section, take a diamond-coated file and gently score the lines you want to cut....
Or just explode it in you hands, like I did.
Taking a couple of the larger sections, the internal surface is pretty much mirrored. Too far away, and there is no image:
But, at just over half the radius away:
An enlarged, reflected image is formed. Since the glasses originally had plane reflectors in, I think it probably won't even need reversing - now to mount them in a test pair of glasses.
NOTE : safety warnings apply - don't play about with razor-sharp wafer-thin slivers of glass near your eyes. Like I am. If I stick glass in my eye it's my own stupid fault. Don't try this at home.