Cross editor plugins are not able to define the keys 'extensions' or 'buttons' in their hook implementations - like implementations of the native plugin hook can - they are assumed to need one button to function.
A good example of where this is not needed is the WYSIWYG Linebreaks module.
This request originated from.
The cross-editor hooks are
hook_INCLUDE_plugin() and the native plugin hook is
These hooks are documented in wysiwyg.api.php and changes to them would be considered an API change.
Though, it might be possible to provide backwards compatibility by first checking for 'buttons' or 'extensions', and then falling back to the regular 'icon path/file/title' keys.