Going to start making every school of grammarian equally annoyed by pluralising "penis" as "penoi".
Going to start annoying the grammatical anarchists by framing a prescriptivist argument for the correctness of "penoi".
as a linguist and conlang appreciator, I want to know what your etymology for this would be
I haven't put a great deal of thought into working out the particulars just yet, but I'd probably start with spuriously asserting that the word "penis" derives from the Ancient Greek φαλλός (note: it absolutely does not) and go from there.
"Grimm's law goes both ways."