I think it depends on the dome. If it keeps the bulb well away from the lip of the dome, then it might be okay to put it right on the top of a screened cage.
However, there's a big caveat here. She has a Reptarium. Maybe they make screen cages, but mine are mesh and I don't recommend putting anything, especially not a light on a mesh top. I say this for two reasons. 1) Eventually they sag and 2) the chameleon can easily push the mesh up quite a bit. If the bulb is 2 inches above the mesh, a good size chameleon on a high perch could easily touch the bulb.
Yes it is the mesh and PVC pipe one, with the zipper. I actually really like it, and it was very cheap but seems of great quality.
What I am going to try and do is install the shelf up high, put the reptarium on it and run an extension cord up the wall, and hang the lamps 2-3 inches above the reptarium with a hook on the ceiling. I think that would work pretty well.