In otherwords, the enclosure could be removed and you would simply have an indentation in the wall that would not compromise the thickness of the overall membrane(s).
As has been said "five siding" enclosures is a commonly accepted approach, however, it is not only the thickness of the membranes, but the the thickness of the space between them that enables a wall to hold up to a test. Depending on the size of the panel, placing it in the wall may cause a localized area where stud support for the sheetrock is spaced farther apart than what was tested by UL. As sandsnow stated, there is no test data that backs up the use of "five siding" to solve these situations
For me, a factor that would weigh in to determining the acceptability of this approach would be, what is the reason the wall is rated? In the IBC for example, some walls, such as stair enclosure walls and exit passageway walls (these are more than just corridors) get special treatment. These are required to have no penetrations except for wiring, ducts, etc., that are needed to serve the enclosure or exit passageway, and that would include both membranes of the wall. I would be very reluctant to allow a panel, even with "five siding", in one of these walls. The wall assemblies were not tested by UL with an indentation. In other rated walls, I would be more inlined to accept the "five siding" approach.
