Matzos and Tefillin of Rabbeinu Tam

Someone presented me with a brand new Sefer called Ot Yatziv, from Zanz, which deals exclusively with the Minhaguim of Zanz in Safrut, with lots of citations from Zanz's most famous Rebbe, the Divrei Chaim. 

The Divrei Chaim says that his Chassidim should only buy Rabbeinu Tam tefillin from a Sofer who actually puts Rabbeinu Tam Tefillin everyday. The reason? The main "drasha" of the Talmud in regards to Tefillin is: 

 וקשרתם... וכתבתם, כל שישנו בקשירה ישנו בכתיבה, וכל שאינו בקשירה אינו בכתיבה (Gittin 45) 

This means that a person who's obligated to put on Tefillin is the person who's permitted to write it, thus excluding woman, children and others from writing Tefillin. 

But the Divrei Chaim uses this drasha further to exclude a Sofer who doesn't puts Rebbeinu Tam Tefillin from writing such a Tefillin, since in this regard he is not "Bar Keshira". Of course, this is a Chumra and a Tefillin written by a non-Rabbeinu Tam Sofer is Kosher. But it's an interesting point. 

 The Ot Yatziv says that this stringency of the Divrei Chaim is related ("לשיטתו") to the famous Minhag of Zanz of not eating machine matzos on Pessach. The Rebbe said that there is a hidden reason for it ("taam kamus") and Zanz Chassidim treat machine matzos like Chometz - that's right, don't try to bring Yehuda Matzos to your Zanz friends on Pessach. (In the other hand, I have a friend who's family will not eat hand-made Matzos on Pessach because they claim that the computerized system of the machine matzos is far more reliable than the hand made process, which they consider more prone to causing chametz. Which begs the question - how did the Jews survive 3000 years without the machine matzos?)

The Divrei Chaim goes so far to rule that if your Minhag is that machine matzos is chometz, like Zanz, you cannot make Zimun with friends who eat machine matzos next to you, because as far as you are concerned these people are eating Chometz - having Zimun with them is a paradox you should avoid. That's the same underlying principle of the Divrei Chaim's chumra in regards to Tefillin of Rabbeinu Tam - you shouldn't buy it from a person who doesn't supports this opinion, since the Drasha of שישנו בקשירה ישנו בכתיבה will not work in this case and will thus cause a paradox to you.

Be it in Tefillin DeRabbeinu Tam, be it in Zimun between hand-matzo eaters and machine-matzo eaters, this is a very big chumra from Zanz. And a very annoying one for people with Zanz in-laws like me...!