Based on photographic comparisons, my specimen closely resembles Isaria tenuipes (aka I. japonica, aka Cordyceps takaomontana anamorph). I am not an entomologist so the closest I can come to identifying the pupa is recognizing it as a lepidoptera. An entomologist colleague guessed from a photo that it is a Hepialus humuli pupa, though the associate fungal parasite (Ophiocordyceps sinensis) does not match the description of my specimen.