Thought I'd check other species in the genus on BG, and just came across this:

I made my initial ID based on appearence, as I know host data for only a handful of species. This certainly handicaps me when trying to identify reared moths! I only noticed the host species listed on the guide page after coming up with my ID suggestion, so I failed to realize other species in the genus share the host. I thought haimbachiana looked best, while substitutionis seemed to have a more contrasing patterning than your specimen. Looking again, I wonder if wear may have led me to dismiss the possibility. I’m just not completely sure. Life history would seem to suggest I may have been wrong, but let’s hear from Charley.

and glad Terry and Charley were able to comment too. These things can get complicated fast! But we can be thankful that examining genital morphology is a thing. If I'm able to get the specimen dissected I'll pass along the verdict. --J

I hadn't heard of that species before. It does seem like a fairly good fit, except maybe for the life history...at least from what I read here it sounds like Gypsonoma haimbachiana is supposed to be a borer even in the young instars (first in vein or midrib, then in bud or shoot). I did come across this reference that claims young larvae "cover themselves with silk mixed with trash, then bore into the midrib." I'm not sure exactly what that means but it does sound a bit like what my larva did with the frass-tubes.

Will move to Gypsonoma haimbachiana tentatively; thanks again :)