A rabbi is putting tefillin on a non-religious Jew. The rabbi gives this Jew his yarmulke to wear. Is it permissible for the rabbi to cover his (the rabbi’s) head with the tefillin bag in the meantime, or do we say that the tefillin bag is designated for holding the tefillin and cannot be used for other purposes or worn over one’s head?
If the person keeps a sidur or something else in the bag e.g. Tefillin marker then no problem. Even if he doesn’t only the inside of the bag has been designated for teffilin.
O:CH 42:3 Mishnah Berurah 11