Furigana may also be useful for not non-kanji characters, such as katakana (and perhaps other things?) as well. Much like Japanese toddlers, I learned hiragana before katakana, so it's a legit use-case in my eyes.
In any case, a quick solution would be "if furigana braces are encountered and the character before is a non-kanji, seek backwards to the next space" or something. It may be worth considering the merits of making this the general case, since it's a necessary workaround when you only want to specify furigana for a few particular kanji. The spaces could be trimmed after the fact.
Furigana may also be useful for not non-kanji characters, such as katakana (and perhaps other things?) as well. Much like Japanese toddlers, I learned hiragana before katakana, so it's a legit use-case in my eyes.
In any case, a quick solution would be "if furigana braces are encountered and the character before is a non-kanji, seek backwards to the next space" or something. It may be worth considering the merits of making this the general case, since it's a necessary workaround when you only want to specify furigana for a few particular kanji. The spaces could be trimmed after the fact.