I came across another Rhopalomyia baccharis gall at the Carpinteria salt marsh today. This one was much newer than the very old, dried gall I previously photographed, though this one had an interesting abrupt transition in the middle, where it went from being green in the proximal part of the gall to being brown in the distal half. Maybe the gall did something to kill the stem at that point?

This image shows most of the gall. You can see the thin stem where it joins with the main part of the plant (coyote brush, Baccharis pilularis) near the top of the image. Then the stem comes toward the camera, turns downward, then curves back toward the camera in the lower right part of the image.

I was able to get a good view of one of the curious lens-shaped exit holes that Russo talks about in his description of the species in Field Guide to Plant Galls.