While bringing on Nehahra support I've noticed that it's maps are positively riddled with textures which have fullbright pixels in the wrong places. Here's one of the worst examples, from the Realm of the Ancients map:
It's occurred to me that one possibly viable approach to resolving this could be to examine the texels in the second miplevel for fullbright pixels, and base the decision of whether or not to load a luma on that. It's purely at the "hey, that's a neat idea" stage right now - I don't know if the creators even bothered to add full mipchains to their BSP textures - and it's fully possible that there may also be spurious texels in that too, which may even be in different places. But at the same time, the filtering down to half size and subsequent blurring, then selecting of a colour from the Quake palette, should give a more accurate result.
I could even go one step further and examine the third miplevel too, then take a wild guess that if 2 out of 3 have appropriate texels we're likely to have a genuine luma texture.
Anyway, I'm going to put this together later on today and see how it works out. More news as it happens!
Edit: done it. It's resolved the issue in the screenshot above, but there are other areas in other maps where the bad fullbrights persist all the way down to the fourth miplevel. Oh well, good enough for now, and at least better than it was.