Sorry to keep bothering with issues, but when I installed K2 2.1 I made three plugins following this example, and they work(ed) fine. However, in 2.2 the plugins break the back-end editing system, it looks like the example image posted with this post.
I'm including one of my plugins - which works in 2.1 - with the hope that maybe I'm just missing a hook here or a call there to make them work with 2.2

I've made some progress...if I remove the conditional "if(count($this->K2PluginsItemOther)" on lines 256 & 258 of com_k2/views/item/tmpl/default.php it loads more of the page. Apparently there is something happening when trying to load the plugin that prevents the complete Joomla header from being loaded, loading only the K2 part of the editor.

Okay, I've hacked my way around this. Here's the (hopefully temporary) solution:

In the file /administrator/components/com_k2/views/item/tmpl/default.php remove the following conditionals in order for your plugins to show up:

LINE 256: if(count($this->K2PluginsItemOther) && implode('',$this->K2PluginsItemOther)):
LINE 258: endif;
LINE 577: if(count($this->K2PluginsItemOther) && implode('',$this->K2PluginsItemOther)):
LINE 591: endif;

This results in the "plugins" tab always showing up and always loading any plugins you may have.