A brief update: I tried breaking the user interfaces out into a set of subplugins but it didn't work.
The documentation here, which
says
they're supported only for activity modules, html editors, local plugins and admin tools, is in fact correct. Code lines 60/61 in component.php provide the proof:
/** @var array list plugin types that support subplugins, do not add more here unless absolutely necessary */
protected static $supportsubplugins = array('mod', 'editor', 'tool', 'local');
So, sadly, if looks like subplugins are not the way to go here, at least not for now.