By Rabbi Berach Steinfeld
Reuven was doing construction on his house. The city required him to build a temporary wall around the site. Along the wall was a door. There is no Torah obligation that would require Reuven to put up a mezuzah alongside this door as it is comparable to a succah, which is called a “diras arai” (temporary dwelling.) Nevertheless, one should put up a mezuzah because of “maaris ayin” (people will accuse him of not doing the mitzvah.) The question arises; may Reuven put up an empty mezuzah cover so he will take care of the maaris ayin?
This idea would seem to be okay as we find similar permissible behavior when it comes to the area of maaris ayin. If someone would like to drink almond milk at a meat meal, the Rema in Yora Deah, siman peh zayin, seif gimmel says that one may drink the almond milk at a meat meal provided that he puts down some almonds next to the milk to show that it is not real milk.
In the event where the milk he wants to drink is not almond milk, but a different type of parve milk, the poskim discuss whether one can put almonds next to it so as to remove the maaris ayin to prove it is not dairy.
We see a contrary view when it comes to Chanuka candles. The Gemara in Shabbos, daf chof gimmel, amud alef says if Shimon lives in a courtyard that has two doorways on different sides, he would be required to light candles at each door. The Ran says that Shimon only makes one bracha; he should not make a bracha for the extra set of candles he is lighting. The Rashba disagrees and says one should make brachos on both lightings. The Rashba compares it to the shechita of a “ben pekuah” (baby calf found in the mother’s stomach alive after the slaughter of the mother calf.) The shechita of a ben pekuah is only required because of maaris ayin. The calf does not really require shechita since it was considered slaughtered in the mother’s stomach; nevertheless, one is required to shecht it and according to the Rashba make a bracha.
According to the Rashba it would stand to reason that one may not put up a mezuzah case without the mezuzah inside since he would not be able to be make a bracha.