U-boot can be told to boot other root file systems on SD card or SATA. This is the limit though, no “live” media exists on CD/DVD in part because the boot loader provides many requirements that a regular PC would have in its BIOS…what a PC or laptop has for providing “standard” boot time hardware query services must be hard coded in firmware files and/or boot loader for most embedded systems.
You can come close by using the existing u-boot and kernel and firmware, e.g., copy the rootfs from the other distribution to an SD card and check the options under the L4T “apply_binaries.sh” script (on your host). The “-r or --root path” option lets you put the Jetson-specific binaries on this alternative rootfs. Then, instead of flashing, you add an edited entry to /boot/extlinux/extlinux.conf, plug in your SD card or SATA drive (the edits point to this), and use serial console to select the alternate boot entry.
One of the reasons this may not be perfect at times is because it runs using the kernel and firmware dtb on the original eMMC, but there is a possibility that your alternative rootfs could depend on a feature not in this kernel. The original Jetson kernel can of course be compiled to work with other features in addition to features it ships with, provided those features actually exist in this kernel (e.g., perhaps a wireless driver exists for a very new kernel but not this kernel because that hardware did not exist until recently…the Grinch kernel exists in part because of the need to back port some popular drivers).