In sefer v’yismah Moshe,Rav Elyashiv was asked this question and he said a mirror is mutar for teffilin since the idea of a mirror has changed and is not considered a tachsit of women nowadays . Interestingly, there is a picture in gedolei yisroel portraits of greatness on page 65 showing Rav elyashiv l’shitoso using a mirror for tefillin.
