The WAN module is connected via Mini-PCI-E connector, and is a half-size card, I think. It does *not* use the PCI-E bus, since there is no PCI-E bus. The connector standard specifies USB connections via that connector, and that is what is being used, AFAIK (re-check by issuing "lsusb" on the command line on your KDX).
Thus, you should be able to replace it with something like this:
http://www.amazon.com/Express-Bracke.../dp/B00MTIOF1Q
I'm not aware if it had been done before, but it seems straightforward.
A thing I would check before going on is how the system reacts to the WAN module being missing.
When you have done the installation part, there are some ways I can think of to integrate it with the firmware:
- try to use it instead of the internal eMMC for the user data store. Could prove to be difficult, OTOH might not be too hard - I'd try with just "mount --bind" of the new block device over the user data partition block device. Or change all the references in the scripts. A benefit of doing it this way would be continued USB mass storage access (well, at least I think so).
- just mount it into the user store. Straightforward, needs just one new line in the boot scripts. Or even make it user-triggerable, say, via a launchpad script. Downside: You can not work on the files using USB mass storage. If you're using your device more "hacker-style" anyway, that might not bother you since you could use USB networking plus SCP/SFTP to access the files.
The latter part needs a bit of insight into scripting on Unix/Linux systems. Oh, and you might need to build a USB mass storage host driver for the kernel. The hardware part seems straightforward.