I'm on my way of doing the same as you for a standard debian/etch, but've encountered some problems (due to my ignorance, I see...)
AFAIK, what we need to accomplish this task is to make a initial ram disk (initrd) with usb support, that loads all the needed drivers at startup, wait until they are correctly active and then mounts the usbHD partition.
When you accomplish this, you have only to tweak your boot loader configuration and add some new lines to boot this new partition (this second one is the easy part).
I'm trying to make the initrd but have little time and experience, should be easy with a baby step guide
. The man page of initrd is Chinese to me...
Let me know your advances, please. I'll keep you informed if I get something.
PS: I leave you some links that help me realizing what the tasks were: