A follow-up: it appears that the best are probably Gossen Luna Pro's, of which there have been many models. Any thoughts on which models to get or avoid? Any problems with fairly old ones? Thanks.

I've been using the Luna Pro Digital for about three years - very reliable and accurate, especially in incident light mode.

The only problem with the older Luna Pro's was that they used Mercury batteries. For a while, Gossen did make an
adapter for two MS 76's which included a resister to make up the difference in voltage. But, I don't know if this is
still availabel.

Camera light meters are quite sensitive anyway, but when the light is very low we are probably into the territory of using "B" with moderate apertures, maybe f8 or f11, and exposing for many seconds or minutes. Just try an exposure, double it, double it again and so on and review the result on your screen. I'm assuming a digital camera! If you are using a film camera use a digital one to establish exposure and then set the film camera accordingly. No need to buy an expensive separate meter IMHO.

True enough that digital cameras make excellent light meters in almost all cases, but I am not sure this is true in really low light situations, which as you may imply remain the province of film. Most digital cameras seem to have limited long exposures, at least compared to, say, my Nikon F3, which will take a pretty accurate exposure of several minutes on aperture priority. But the camera I have in mind is a Mamiya TLR. I don't like extra exposures with it, not only as they're fairly expensive but because of the time it takes to deal with the images.
Of course I may be wrong about the long exposure abilities of a few very high end digital cameras.