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For those who run across this issue in the future, I managed to solve the problem, more or less, by inputting the Japanese directly through the filemanager in control panel. Doing so changes the characters into unicode [eg の] which means they'll display okay on the page but are indecipherable in HTML view, making them uneditable.
These are the kinds of issues I'm coming to despise more and more about XOOPS in particular and PHP in general: what a colossal waste of time. It all too frequently takes 5 or 6 hours to overcome something that could be dealt with in a matter of moments in straight up HTML. If I can find an easy way out, I'll take it.