"I have now a copy of a letter sent by the Pontifical Commission “Ecclesiae Dei” and signed by the Secretary Msgr. Guido Pozzo, which clarifies that UE 28 in fact does say that females cannot serve at the altar in the Extraordinary Form.
The letter essentially quotes UE 28 and then states:
In this regard, the Circular Letter of the Congregation for Divine Worship and Discipline of the Sacraments of 1994 (cf. Notitiae 30  333-335) permitted female altar servers, does not apply to the Extraordinary Form.Bottom line: females may not serve at the altar in the Extraordinary Form..."