On this I do not agree with you, although MyRoundCube has a lot of additions to the regular one, other features or not available, like selecting a different skin.
You are a template coder, so I understand your wish. But you are only paritally right.
At the moment I have disabled skin selection (./myplugins/config/config_plugins.php[.dist]):
/* api template */
$rcmail_config['skin'] = 'api';
/* Do not allow override template on a per user level */
$rcmail_config['dont_override'][] = 'skin';
But theoretically it works with all skins if the skin contains appropriate MyRoundCube XML.
F.e. to insert calendar task icon it is something like (I dont't have the real function name in mind right now):
So it is easy to adapt the calendar plugin to another skin.
I just don't catch the time to write a documentation for my Plugin API ...