Well, the moon and Jupiter aren't ionized the way stars and HII regions are, so there isn't a clear relationship between temperature and brightness.
You do raise the point, however, that there are baryonic components of dark matter. We have two handles on that. First, we can measure the percentage of baryonic matter in the universe by looking at the abundances of light elements. Second, we can search for dark, compact objects (presumed to be baryonic in nature, such as "brown dwarf" stars) by searching for "microlensing" events, where a compact object passes in front of a single distant star, with the gravitational lensing of the object causing the star to brighten temporarily, in a characteristic way.
The upshot is that, while there is a surprising amount of dark, baryonic matter in our galaxy, there isn't nearly enough to account for all the dark matter.
If the light rays entering a telescope are indeed parallel for images of distant objects, then that implies the light path is simply a cylinder equal to the width of the telescope aperture (neglect sidelobes). Is it really realistic to use the ensemble gravitational field of a cluster "causing" the bending of the lights path? Wouldn't it be more accurate to use the highest gravitational potentials the path crosses? Doesn't the gravitational field in a bend causing cluster vary widely across the cluster? I could shine a light through our solar system and come up with many different values for the bending depending where the light path was - near Jupiter, near the sun, near Pluto, etc. There is no uniform gravitational potential in our solar system and I don't think it is uniform across a cluster that bends light.
So couldn't the dark matter be really making up for the assumption that a bending cluster acts as a point gravitational source rather than a non uniform distributed one, which it really is?