I have a number of meta boxes added to a site I'm building, and would like to have their data saved via the REST API rather than have the post save "twice" in Gutenberg (once via REST for the basics, then via POST for the meta boxes).

I can't really convert these to "meta blocks" for various reasons, and I don't want them buried in a plugin panel. Besides they're built in PHP already and it'll take too long to recreate them as React panels.

Is it possible to register these meta boxes as REST compatible or otherwise hook into the action that makes the REST request so I can add my data there (after registering REST support).