A closer inspection of the bush revealed at least two more of the same species, though that took me no closer to identifying the creature. I snapped all the pictures I could, intending to iron out the who's-who questions when I got home.
A couple of pullouts later, however, the main feature was several currant bushes that had been most definitely settled by the same sort of caterpillar; their manner of settling suggested very strongly that they were tent caterpillars, or Malacosoma californicum, if you'd rather.
I'm still rather curious as to why I found a scattered few of the creatures, unhoused, in the sulfur buckwheat bush. They are evidently pretty gregarious.

