Wasn't his original name something like Mindhu, or am I completely off-track here? Like you said, the it's kinda slim pickings when it comes to the characters, but Minwu is a good choice.
His Japanese name is spelt MinU, as in "Mi-N-U".
This implies that his name is supposed to be MinWu, because if they wanted it to be "Mi-Nu" then they would have used the "Nu" syllable, and the U syllable is used for "wu"-sounds in Japan.