Starting around OWRT 19.07, we have a new installation method that involves an initramfs-factory.tar, available from here, installed with the EdgeOS Web-UI. Another - method, though I don't know if this is still viable, uses the serial interface to tftp the initramfs-kernel.bin, available from the Firmware Selector. Either method is followed by sysupgrade applied to squashfs-sysupgrade.bin.
I would rather use the serial port / tftp, as once RAM booted into the initramfs-kernel.bin, I would be able to back up the stock partitions. Is it still viable? (But on that backup, see my next question.)
I had missed that the details of how to flash with tftp are actually here. What is not described there is that after the initramfs-kernel.bin boots in RAM, we need to switch the laptop connetion from eth0 on the router to eth1. For this to work, on initial boot into RAM using tftp, we have to give the device and laptop an ip in the range that openwrt, running in RAM, will later be giving to itself and via DHCP: