Many years ago my family and I did colonial re-enactments- wearing period glasses was something my father chose to do and invested a lot of money into.
The problem is that despite the glasses being period- he spent a lot of money on authentic frames- they still looked more modern then anything else on his attire; a consistent problem I saw with most Revolutionary War and Civil War era re-enactors.
For Dag, I find it similar, though I can see some things people can choose to do in order to 'hide' the modern glasses. The big one is the people who have sports goggles and still have the pretty obvious elastic headband. I would imagine that one could wrap it in dark cloth or even add a layer of thin leather or something to hide the elastic band. Or cutting off the toe of a small black sock and pulling it over the band as a cover.
Metallic paints- brass, bronze, iron-looking metals may do the same for frames, but if it's too obvious, it'd end up looking silly. I had a pair of glasses from my time in the military I painted metal, they looked /okay/, but for dark fantasy, it's better then sharply obvious plastic frames. Sadly, those were eaten by a grue, so I have no pictures.