Goes to show I should think a little harder before trying clever solutions like directly editing memory. I was basing my process off the techniques applied in this thread.
I got past the kernel panic issue, which I think was actually the result of incorrectly-configured TFTP settings. Now I've got a working initramfs image, but attempting to load and write a squashfs image just causes a reboot with no apparent changes. Here are the full boot logs including the U-Boot portion.
initramfs Boot Log
squashfs Boot Log
Luckily the byte I edited was followed by a . (0x20), so I don't think it caused that large of a problem.