Intel should be making an Optane sdcard for embedded use, potentially with a built in power down capacitor to make sure all acknowledged writes make it to Optane.

When using netboot, the boot rom will pull the files it needs from the tftp server. It will first look for them in a subdirectory with the Pi’s serial number as its name.

If you’re netbooting without any sdcard installed, I’d suggest disabling sd polling.

PXE uses the MAC address doesn't it?

It does, but I think what the Pi does is connect to the TFTP server handed out by PXE and then look for a folder named serial-number. If it doesn't exist in the path PXE gave it it just boots whatever is in the tftproot.

Might or might not. It does DHCP request and server-side, you can do decision on what it returns on any tag you want, often client architecture (0x93); you can then point to the right tftp server, or if you have UEFI client, you can also use http.