I'm in the process of converting existing XPC C code (from another developer) to NSXPC Obj-C code. So I can't say and have no idea whether it ever has been seen by the original XPC C code.

On the server side.

As far as I understand, once loaded and user approved, your network extension will be started automatically on the next restart (which was not the case with kernel extensions loaded manually). The issue is actually more with the companion app/process if this is a Mac App Store solution. For the companion app/process to be started at boot time, you would need to use a launchd daemon. And launchd daemons are persona non grata in the MAS AFAIK.