I would like to use my Linux host to update and upgrade a Raspian image. Can this be done?
I know you can mount the image and make permanent changes, see e.g. https://raspberrypi.stackexchange.com/a/13138/101680
The reason I want to do this is I have a Pi Zero that won't boot. I suspect a bug in Raspian Buster and would like to experiment with the latest changes (Pi Zero USB On The Go cannot connect)
You should be able to use Qemu User Emulation to accomplish this via chroot. Its a similar sort of setup to debootsrapping a foreign arch but you'll already have the OS installed.
This assumes your host is Debian based - i'd assume the Qemu stuff is available on other distros though.
Important steps from the Debian Wiki
sudo apt-get install qemu binfmt-support qemu-user-static
Mount your Pi SD card somewhere on your linux host
mkdir /tmp/pizero
sudo mount /dev/mmcblks0p1 /tmp/pizero (or whatever it is called on your system)
Copy the Qemu binary onto the pi SD card (can be deleted when you've finished) - might not need to do this anymore though.
cp /usr/bin/qemu-arm-static /tmp/pizero/usr/bin
Chroot into the Pi and you can then treat it as if it was running natively.
sudo chroot /tmp/pizero
and you should end up as root on the Pi and able to run apt and stuff against the Pi FS.