Well, I may answer my own question here.
This post says initrd is the only means to directly flash nvme, and that requires a fully working USB-C 3.0 port on the target:
This provides some manual steps to provision a rootfs partition on an nvme drive – but the drive has to be removed from the SOM and connected to the host. The same steps could likely be done on the target, if the target can from emmc and it’s rootfs has parted/gparted and rsync – the rsync could be done over TCP/IP. Pretty painful.
https://docs.nvidia.com/jetson/archives/l4t-archived/l4t-3261/index.html#page/Tegra%20Linux%20Driver%20Package%20Development%20Guide/flashing.html#wwpID0E0QN0HA