If you don't have working "built-in" Ethernet (or your computer has no Ethernet at all) and do not have a supported PCIe WiFi card, then chances are you're using USB WiFi for internet access. The problem, of course, is that you have no access to the Mac App Store in that scenario.
On a whim, I created NullEthernet.kext to solve this problem. It is a "fake" Ethernet kext that will never actually work. That is, it will not ever provide network access, but it can be made to look "built-in" and solves the issue of Mac App Store access.
For Source/README/download, see here: https://github.com/R...X-Null-Ethernet
It is now confirmed working in at least two laptops where there is no Ethernet port, and the WiFi is unsupported (and not replaceable).
A new build is up. This version can be attached to a PCIe device as well as a fake ACPI device (as in the first version). This allows you to load the driver against an actual non-working PCIe device if that is your scenario. It also avoids a DSDT patch as all customization is done in an injector kext by modifying its Info.plist for your device. The injector/PCIe technique is not relevant in the scenario where you have no Ethernet device at all, for that case you must use the fake ACPI device. See the README.md for details.
A new build is up. This version can be installed without any tweaks to ACPI, and even without a PCI device to attach to. Simply i install NullEthernetForce.kext along with NullEthernet.kext. Read the README for further details.
NOT WORKING YET. REMOVED.