Semi-Private Comments + page comments -> is it possible?

Sure it is, it is open source so you can hack it anyway you like as long as you know how to code 😉

There are three ways I see to do this:

  1. Find a plugin that does this
  2. Write (or modify a existing) plugin to reach this
  3. (Bad) For your template edit comments.php in order to reach the only see my own comments (or show all comments to admin), for the twentyeleven theme you would also have to edit the function twentyeleven_comment that is in functions.php

The 3rd option is bad as in not recommended as that makes it harder to update/upgrade in the future.

Obviously 2 and 3 means that you need to mess with the code or hire someone to do it.