I'm curious if the Pi Zero is small enough that you could actually fit it fully inside the CM15 case? If there was a way to embed it for a compact and clean installation, this SHOULD be a no-brainer for anyone who has a CM15 and is interested in some real capabilities beyond what AHP can provide.
I understand that there are other components that may be an issue for this setup (5v power supply for Pi needed, internal connection for USB on CM15 not directly available, etc). But, if the Zero doesn't fit then it's already a no-go.
Depending on the age of the CM15 one plans on using you could fit the Power supply and Zero W inside the CM15. You can wire the power directly to the Zero which I plan to do in another mod. It also may be possible to steal power from the CM15 circuit board itself if you can determine a 5 volt run with enough amps.
the newer Cm15A boards have many components incorporated into the board and the Zero W only takes the bottom half of the battery compartment. see my write up here:
http://forums.tuicemen.net/index.php?topic=1291.0Since the Zero fits in the one half of the case you can swap it out with a new or old CM15A.
my mod was originally done with the old Cm15a which had RF daughter boards limiting the placement of the Zero.
I've since moved the moded section to a newer CM15. This has space to fit a separate 5v power supply above the Zero.
the only wire sticking out of the Cm15 case that would be required is the USB connection to the two. These can be ordered in white and only 6 inches long from Amazon.