Display overlay informations in the edit area

Hello,
I need to customize the editor allowing to display custom informations in the edit area (like baloons, tooltips, etc.). These informations may contain custom links and buttons to operate on the text (whole or range).

I managed to do part of it as a plugin, using CSS: some hidden tags are created, containing the links I need (still didn't try with buttons), that I display when necessary. But being part of the current text document, their content is still modifiable!

A first approach would be to "lock" the content of a certain tag, preventing the editing of only its content. Is it possible?

A second approach would require to have an overlay layer, not editable, where to display my custom informations.

Can someone help me, please?

Thanks very much
~Aki