I came upon this caterpillar crossing the road - obviously prepupal. Eacles imperialis is the closest match I can find, although it differs from the description in Wagner in a couple of aspects - yellow spiracles instead of white, and slightly larger - I measured it at 9 cm in an unstretched position - he gives 8 cm as the maximum.
Unless anyone can suggest a good alternative, I'll file this as Imperial Moth and chalk those differences up to its prepupal condition.