Tooltip plugin

I would like to write a plugin to modify the tinyMCE editor. The plugin would add a new icon (button) to the text editor that would allow a user to highlight a word and click the new button. The click would display a modal popup (similar to the URL modal) that contained a textarea and submit button. The user would then be able to enter text into a modal textarea that would save the content of the tooltip.

My question is, does this plugin already exist. If not, would it be better to write a plugin for tinyMCE, rather than wordpress?