I’m very much for this. It could lead to much larger adoption of Markdown outside English-speaking communities, even.
I do have some thoughts about the syntax (as I’m sure many others will). Since Markdown is inspired by common plaintext practices, and is supposed to be minimal, syntax like [図](-と) is… well, a bit odd, to say the least.
StackExchange’s Japanese site has a fairly well-executed implementation of ruby tags implemented site-wide. It allows three different syntaxes for ruby tags: [漢字]{かんじ}, 漢字{かんじ}, and 漢字【かんじ】. Out of these three, the last one is the most similar to what people use in plain text. In fact, it is exactly what people use in plain text. The first syntax has the advantage that it’s unambiguous, which is required for putting ruby tags over anything other than an entire contiguous group of kanji.
Personally, I’d suggest the [漢字]{かんじ} (unambiguous; easy to type) and 漢字【かんじ】 (already widely understood; low-friction) syntaxes, perhaps with the addition of [漢字]【かんじ】 for consistency.
I should point out that the StackExchange implementation has a few bugs and oddities (there’s no escaping; ruby can’t be put above non-CJK characters), but as a reference point it’s very nice (and has already been put to good use).
Also, this makes me think… Wouldn’t it be beneficial for CommonMark to support full-width brackets (［］（）｛｝【】) in addition to normal ones? That way, it would be immensely more friendly toward CJK text, which most often uses full-width punctuation.