Rooting on non-boot device GPT partition

[John Klos <john%ziaspace.com@localhost>]

On older Pi systems, rooting on any other device & partition was as simple 
as editing cmdline.txt and setting root= to whatever we wanted, be it 
sd0a, dk0, whatever.

I see the default entry there now refers to the GPT partition on the image 
(netbsd-root), but changing that doesn't do anything.

Also, we can't compile a kernel with NAME or with dk0.

I COULD make a hybrid MBR / GPT with root on the MBR which is in the first 
two terabytes of the disk, but that seems kludgy.

What's the proper way these days to root off of a GPT partition on a USB 
adapter on a Pi 4? Or any other system?

Thanks,
John