I don't know if someone has already spoken about that.
If you check in the character editor there are some regent of Mieres with "unknow" state because Mieres was cut.
Well,if you made a scenario (you don't need to do any modification) you can play with Mieres!You have only to click above the button of "begin rule" (NOT THE BUTTON),of the second ruler (for example Braedonnal Tuare),you will collect the taxes of the selected realm and POUF you will rule as Mieres.
As Mieres you have no land and you simply can't act,but you can watch the AI fighting without interfere.