Attempt to condense this into a rule:
Songs with Japanese titles must use the Modified Hepburn method of romanisation in the "Romanised Title" field. As a non-unicode field, long vowels such as "おう" and "うう" should be romanised into "ou" and "uu" to avoid macrons. Loan words should be expressed using romanisation from the original language.
Another potential issue is this: if the creator of the song has provided a Nihonshiki or other alternate romanisation, should this be used instead? People may say yes because it follows the intentions of the creator, but people may say no because it creates inconsistencies and still does not count as the "original" title that is being romanised.
Like with the custom difficulty name thread, I would like to suggest to Melophobia to create a tentative deadline for finalisation and update the rule with suggestions from others in the thread.