Is there any way to get at a OneNote page's custom (user-defined) tags via the API? (I'm aware they don't appear in the HTML content.)
(The Evernote API allows you to read (and set) custom tags, which makes it considerably more versatile.)
sorry we don't have support for custom tags via the APIs yet. Full list of supported tags can be found at https://gist.github.com/nickbarnwell/c695fc4bb5b34d998622.
Please consider voting up for custom tags at UserVoice