Kindle Previewer is notoriously unreliable when it comes to displaying complex non-Latin text. You may want to test your Japanese book on an actual Kindle, because even my old K3 displays horizontal Ruby text fine in spite of KP saying it doesn't.
IMHO, it's safe to assume that all current eInk Kindle models (i.e., K3 and higher) support at least horizontal Ruby text.
Spoiler:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<style type="text/css">
body {font-size: 130% }
</style>
</head>
<body>
<h3>Simple Ruby test</h3>
<p><ruby><rb>東</rb><rp>(</rp><rt>とう</rt><rp>)</rp> <rb>京</rb><rp>(</rp><rt>きょう</rt><rp>)</rp></ruby></p>
</body>
</html>
BTW, what kind of CSS did you use to format the vertical Ruby text?