It did strike me as having a Hemiptera/Homoptera type mouthpart, but I (obviously) don't know much about mites (other than that at least some have a 6-legged nymph stage), and thought some might have similar mouthparts.
The surface of the 'cocoon' under magnification had a tightly coiled appearance that I interpreted as silk, but I suppose the coils could be wax filaments--I'll attach a photo in a moment.
Looks a lot like this Pulvinaria egg sac.
I never would have thought of scale insects, since this was all by itself on the stem and there was no sign of a female's scale covering or anything. Guess I need to do some more reading about this group!

Check out cottony-cushion scale for an example of a scale that isn't a hard-shelled type.

In my case the remains of the female, hard-shelled or otherwise, were nowhere to be found.
So far Pulvinaria scales are the only ones I've read about, or seen images of, with egg sacs resembling this one--the cottony-cushion scale has those distinctive longitudinal ridges--but I recognize that there are so many types of scale insects out there that I'll never know for sure.