I don't know much about Cacko, but I eventually worked out how to get the kexec-boot thingy working, and I'm glad I did it is bloody ace! Forgetting Cacko or NAND for the moment, I will explain what I have learned about kexec-boot.
Once you have flashed the Z it has a special kernel which when it boots, will look at all the partitions it can find (including those on attached media) and if it finds any with a /boot containing a zImage it will present them in a menu.
So, on my Z right now I have that special kernel. I have made two partitions on the microdrive.
In the first I extracted a recent Angstrom root filesystem, and then in the /boot I placed the Angstrom zImage file, and made the kernel-cmdline file containing:
"console=ttyS0,115200n8 console=tty1 noinitrd rootfstype=ext3 fbcon=rotate:1 dyntick=enable debug"
The image.nfo file doesnt work for me. I have no idea - I think it just prints some pretty labels (which would be handy) but things work without it!
Then on my other partition I extracted the zubuntu root filesystem and the omegamoon kernel, and an identical kernel-cmdline file (It may not work for other kernels - Im thinking of the Cacko kernel here specifically - its 2.4 isnt it?).
So basically, when I boot my Z I get a nice menu with those two.
Now the cool thing is, you can install Debian / Angstrom / Zubuntu or whatever to a CF or SD card, and boot from that to install stuff on the microdrive (Cortez alludes to this in his "how to" to get Zubuntu installed on the flash).
Anyway, having covered all of that, I will now speculate wildly about things would work with Cacko.
I am guessing that the process goes something like: Flash Cacko normally. Either then boot into Cacko and somehow get a copy of its kernel, and place it in /boot with an appropriate kernel-cmdline on the NAND where Cacko is installed, or flash kexec-boot, and then boot from a distro installed on SD or CF fixing up /boot as described, either way flash kexec-boot after you have flashed Cacko. You can then boot it because it should show up in the kexec-boot menu. You would then be able to put whatever you want on a microdrive (or indeed on a SD or CF card should you have a Zaurus without one).
Hopefully that will help!
(And hopefully someone can fill in any gaps on my knowledge - I am kind of tempted to try getting an English translation of the Sharp ROM on my NAND).