I would argue that the gender of this subgenus is indeterminate. I would suggest using whatever has been general practice for now, and making a Case to the commission about it. If you want a general hand-wave gender to use, I think the best bet is masculine.
The original incorrect spelling is
Pseudoön. Under 32.5.2.1, the diaresis is deleted, as this word is clearly not German in origin. I don't see any way that this mark could be considered an umlaut; it's clearly not Germanic (or any other language except Greek). It's much more likely (in my mind) to have been used as a term to show that ön is a separate word and pronunciation from the first ending -o. I can't think of any other instances where an ending is applied with a diaresis like this. This makes
ön likely to be a word in its own right, and it's likely to be either
ὤν (masculine) or
ὄν (neuter), participles meaning 'being'. I can't find instances in dictionaries of these being substantivized, but it really doesn't matter much - they're valid Greek words, and they won't change ending as they're generic and not specific. Both of these Greek words could be transliterated as
on into the Latin alphabet. There's no clauses in Article 30 that say what to do if a final word is Greek and can be interpreted as either masculine or neuter - it does mention "common" or "variable", but then limits that to masculine or feminine, which is unhelpful here. The language has to be interpreted as Latin or Greek under Article 26, so Article 30.2 can never apply, which would say that anything ending in
-on is neuter.
The spirit of Article 30.1 is that names which are unclear default to masculine. I don't like using the spirit of the law as a reason for clarifying gender, at all, but that's probably the easiest route if you need a short answer. If people have been treating it as neuter, use that.