Monsignor Guido Pozzo, Secretary of the Pontifical Commission Ecclesia Dei, has written a letter confirming that the permission for females to serve at Mass, granted by the Congregation for Divine Worship in 1994, does not apply to the Mass when celebrated in the Extraordinary Form.
No altar girls at the Traditional Mass. It was clear before. It is clear now. It is also clear that the PCED is willing to move extremely swiftly to clarify these matters.
Here is the letter.
I've written about why altar girls are not appropriate at the Traditional Mass on this blog: introduction, the problem they present for vocations, and the underlying theological issue.