Well, I managed to squeeze an extremely stripped down copy of IZ2S onto the jffs of a stock zipit, with rockbox, dropbearmulti, and the retawq web browser. This means you can boot the zipit with no SD card and get on the internet in a pinch (kinda, sorta). As you can see in this (somewhat blurry) picture of retawq browsing this very blog, with nothing in the miniSD slot.
To make things fit I stripped and upxed all the executables. The jffs does it's own compression, but upx compression is slightly better. I also removed just about everything I could - fonts, languages, themes, plugins, libs. You name it, it's gone. There's about 800K left on the jffs if you find something missing that you just can't live without. I made softlinks for the rockbox plugins directories, so you can play them off the SD card if you really want to. Static linked executables from IZ2S can also run off the SD card, although if you want that, you should probably just boot into IZ2S itself.
One little gotcha trying to put this together was the IZ2S tar program. I couldn't get it to *create* an archive. Only untar seems to work. So I had to relearn cpio to get test builds off the jffs with my softlilnks intact. I haven't used cpio in like 30 years...
Anyhow, here's how to install. Put iz2jffs.tar.bz2 on an IZ2S SD card and boot it up. Then you want to delete just about everything on /mnt/ffs except the start.sh script, the wpa_supplicant directory with the wifi drivers, and the properties.txt file with your MAC address. After that you can unzip the new stuff, power down the zipit, and boot it up without the SD card.
tar -xjpvf /mnt/sd0/iz2jffs.tar.bz2
The SD card (with all your music) shows up in Rockbox as Files/share/sd0.
Update: I've been fiddling with the busybox from iz2s enhanced because it has so many goodies. It's almost 600K upxed, but it may be worth it since you can get 100K back by replacing the full sed and netcat utilities in /mnt/ffs/bin. I've also compiled and upxed tinyirc (37K) and I'm still trying to decide if the upxed dialog utility is worth it for the wifi script at 150K. Here's a zip with these if anyone else wants to play.