Good news:

rpi-imager --cli

Has really expanded options over the previous versions.

Including:

--first-run-script <first-run-script> --cloudinit-userdata <cloudinit-userdata> --cloudinit-networkconfig <cloudinit-networkconfig>

Though it doesn't support the old simple config text file, presumably most of that is replaced by cloudinit network config.

I am guessing this is available in the Windows version. I am seeing this from the macOS binary.