I lately have limited options for immediately available devices, so I wondered whether it would be possible for me to upgrade to Crimson with an SD card.
I started by looking at the contents of librem5-flash-image script that the reflashing guide in the wiki refers to. The script uses a hardcoded URL for the images repository at https://storage.puri.sm/librem5/images/. After inspecting the repository, I downloaded the latest image with wget and unarchived it. Having the image, I powered off Librem 5 to insert a spare SD card, unmounted it after booting, and copied the uncompressed image to it. It is similar to creating a bootable USB flash drive for desktop distribution. To boot from the SD card, hold the volume down button, press the power button, and then release the volume down button when the LED glows green. The green LED indicates that the U-Boot is booting. The partition is resized automatically during the first boot to occupy the free space available on the media, which is the SD card here.
Now, when operating from that SD card, I expect to be able to repeat the same trick and write an image to the eMMC. That is something I am going to try when Crimson is declared stable.