instead of cat, install picocom and run:
picocom -b 115200 /dev/ttyS0

Mark - would it make sense to put something (udhcpd) in your bigdisk upgrader image to configure the network?
What is in the bigdisk images and can they be rebuilt from source? (and maybe bblobs)

Tony - won't 'download' work from linux (a tiny .c program that installs a kernel image via serial at the "penguins" bootloader prompt?)