Adding Custom Actions to TinyMCE icons

I’m wondering if anyone has done this or can point me in the right direction... you know how you can click the little Tree icon in the content box and you get a nice dialog box for inserting an image into your page? Imagine something like that which allowed you to insert your own custom media. I’m guessing this has been done somehow.... e.g. if you had a database table listing video assets, and when you clicked your custom icon you could search for the asset you wanted to use... once selected, the output would formatted into a nice embed tag.

Anyone seen this? Anyone done this in MODx? Thanks!