A bit of work has been done on this in the past. Initially it was by process of elimination; this has been adequate for differentiating genera of onychoteuthid (Onychoteuthis, Notonykia/Ancistroteuthis [a few problems here; we're not saying that they are synonyms at all, just that there's a bit of work required to accurately differentiate them], Moroteuthis etc.), and we followed this up with DNA sequencing. We know for sure what larval morphologies can be attributed to Onychoteuthis 'banksii', Ancistroteuthis/Notonykia sp, and Moroteuthis ingens, but not yet for M. robsoni.
I'm probably being a little naughty using the generic name 'Moroteuthis', as the TOL refers all Moroteuthis species to Onykia; call it force of habit or precaution rather than us having information to the contrary.
How one differentiates larval M. ingens from M. robsoni (and yet another indeterminate species in NZ waters) on morphological grounds has yet to be determined, if in fact it can be done. There are a number of 'varieties' of Moroteuthis larvae in the collections - some slender, some stumpy (like the typical Onykia form), and these could prove to be separate species (and the larvae of particular Moroteuthis species); they could equally prove to be different preservation/fixation-induced morphologies. Nothing is certain at present; it's part of Kat's research project.