After thinking about this a little more, a different plan comes to mind: Furnish the box to client, and ask that they have their tile contractor handle the mounting. Just make it clear that the final installed unit will be fairly heavy, and that you need a hole behind/through the box for cabling. I would pre-drill the box if required, and maybe furnish a short stub of PVC for the wall penetration.
This puts all the tile-related aspects in the hands of the tiling contractor, which may be better than getting into a conversation about expansion gaps and sealers that could be perceived as telling them how to do their job. Besides, they may have a recommended method of mounting something like this that I have never even heard of!
If the customer hires a trunk-slammer tile guy, and you come back to find the box stuck to the wall with duct tape, then I would just get them to sign a release [I know, that is easy to say now, but in reality...]
Jim
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Pay no attention to the man behind the curtain.
