It's possible to install Rockbox into the firmware partition, and then patch it with a modified bootloader to allow dual-booting. This gives faster Rockbox startup at the expense of slower original firmware startup.
Save the original firmware to apple_os.ipod:
ipodpatcher -rf apple_os.ipod
Put that in the root directory on the iPod.
Write rockbox to the firmware partition:
ipodpatcher -wf rockbox.ipod
ipodpatcher -a bootloader-ipodvideo.ipod
You might want to save a copy of the firmware partition in case something goes wrong, but you should be able to restore from the apple_os.ipod file. If you put the original firmware back in the firmware partition, you probably don't want an apple_os.ipod file in the root because the Rockbox bootloader will load it instead of running the already loaded firmware.
Here's a link to the patch and a pre-built 30 gig Video iPod bootloader. This bootloader should only be used for this special application.
(I was trying to figure out how to load the original firmware from within Rockbox. I was able to find some hardware initializations which prevent it from running properly, but I wasn't able to undo those. So I gave up for now and did this instead.)
Edit: Custom bootloader mentioned above is obsolete. The required functionality is in SVN since r20921, via http://www.rockbox.org/tracker/task/9730