There's a Lubavitcher in my shul who takes off his tefillin during the middle of Shacharit, and puts on another one. I asked him what he's doing - why is he putting on a different pair of tefillin.
He says, "I'm wearing Rabbeinu Tam's Tefillin." I asked him, "Are you sure that Rabbeinu Tam won't mind? Does he know about this?" He says, "No, he doesn't know about this, but I don't think he would mind at all."
I don't know about how "kosher" this is. It sounds like stealing to me to take Rabbeinu Tam's tefillin without his permission. Not only that, you would think that he would return the tefillin to the locker that he borrowed it from. But no - he takes Rabbeinu Tam's tefillin home with him!
This really sounds like stealing to me. The guy tells me that many Lubavitchers are doing likewise. Are they all stealing? If they get caught, wouldn't they have to pay a double penalty?