Before we started using MB, I'd been creating a Metabox Grouper class to combine related metaboxes or custom fields.
Turns out, MB has everything I was thinking of adding and more, but it's missing one tiny thing...
When I inserted a standard WP custom taxonomy, rather than create my own drop-down or select, as you've done, I simply called the WP functions:
post_categories_meta_box() and post_tags_meta_box()
which inserts the standard WP interface for adding terms. This has the advantage of allowing users to add new terms without leaving the 'Add new post type' screen.
Could you add another option for the category and tag metaboxes to output the standard WP interface, as well as all the options you have?
Unless of course I'm missing something that would allow users to insert them from the post-new page already?
I'm mainly thinking of when you want to have a taxonomy field in a metabox with other custom fields or taxonomies...
The main reason I started doing it is because I'm currently writing a plugin that has a LOT of taxonomies and custom fields for a custom post type, and the client wanted to group together related elements so as to avoid having so many metaboxes.
And it was really straight-forward using the functions I mention above. It would be useful for my purposes, but I can see it's a bit of an edge case 🙂