The rhododendron is an evergreen ornamental--I'm not sure of the species name, but it looks like a giant mountain laurel with showy rose/purple flowers. Pretty different from R. nudiflorum, but perhaps similar enough from a beetle's perspective. I was struck by the similarity of these larvae to the introduced Pyrrhalta viburni larvae I see all time, so the ID you're suggesting would make sense. Unfortunately I never saw any adults around.

A number of native rhododendrons are deciduous but the genus is based on flower characters I believe and not whether the plant is evergreen or not. Although Mountain Laurel is in the same family the flowers are fairly distinct looking. But I'm digressing. Most if not all chrysomelds overwinter as adults so you stand a good chance of finding some. Chrysomelids have a fairly short pupal stage, often emerging within a week or two of pupating. If you get a chance to go back to the spot you might just solve the mystery.