In reply to Steve.

Can TinyMCE be customised for users?
[...]
Custom post types

Keep in mind that this is only a 1.0. Although WordPress.com and the desktop app don't support Custom Post Types and Custom fields just yet, it may be added later, in future versions. You can follow both of these suggestions here:
https://github.com/Automattic/wp-calypso/issues/587
https://github.com/Automattic/wp-calypso/issues/588

It doesn't look like anyone suggested custom TinyMCE buttons just yet, so do not hesitate to open your own issue here!