Honestly, anyone who is sufficiently capable in japanese to know that 港南 is something close to "Ko + nan" is going to know hiragana and be able deduce the onyomi readings of each (or at least, of 南, which is the most common reading of a basic kanji), and will not need romanization. The real value of the romaji is for people who know nothing of the language, and it's just weird that the common romanization leaves these important vowel details out.
(I'd personally love a unified syntax for tone that doesn't involve drawing squiggly lines over the word, or a number next to it, but that's a pretty specialized need, I think.)