Thanks for your suggestions guys.
I like the idea of grouping sectors under a single tag based on a light level. However, it would become problematic over a larger map due to sectors requiring unique effects such as coloured lighting (as HotWax pointed out). Also, it would fall short when trying to determine the brightness of an actor
, which is what my original suggestion was, as it wouldn't account for things like fullbright sprites - like a Lost Soul in a 0-level light room, for example. And there's still the problem of GZDoom lights...
For my own purposes, I could hack around the fullbright problem by calling an ACS function from the actor's DECORATE which 'flags' it as having a light level of 255 where needed. All I need now is a big [Yes] to GetCurrentSector(tid), and I can go.
Thanks for taking an interest.