So finally I got it working! The Hentges ROM on my Zaurus SL-5500G is up and running. It was a little bit crazy because it always hung during ipkg <somewhat>, so I show you how it works ;)
For the whole procedure you need one SD card and your Zaurus.
1. Get the Hentges GPE rootfs image (you also can choose Opie, that does not change anything). You can get the cardfs images too, then you will have more apps after installing.
2. Clear your SD card and create one ext2 partition on it (yes, ext2, ext3 didn't work for me).
3. Extract the rootfs.gz to the boot-images directory on your SD card (it should be about 150MB :/) and rename it to $FOO-rootfs.bin where $FOO is some foo ;-)
4. Put the SD card into your Zaurus and boot from it (altboot option #3).
5. While booting you'll be asked if you want to have your ipkg-lists in RAM. Say no!
6. Setup networking and ssh into your Zaurus.
7. If you have booted the GUI, stop it: /etc/init.d/gpe-dm stop.
8. As Hentges has changed his feeds since the last image-release, you need to update the ipkg.conf. Mine looks like this:
src upgrades http://ewi546.ewi.utwente.nl/mirror/ www.openzaurus.org/official/unstable/220.127.116.11/feed/upgrades
src machine-upgrades http://ewi546.ewi.utwente.nl/mirror/ www.openzaurus.org/official/unstable/18.104.22.168/feed/upgrades/machine/collie
src machine http://ewi546.ewi.utwente.nl/mirror/ www.openzaurus.org/official/unstable/22.214.171.124/feed/machine/collie
src base http://ewi546.ewi.utwente.nl/mirror/ www.openzaurus.org/official/unstable/126.96.36.199/feed/base
src x11 http://ewi546.ewi.utwente.nl/mirror/ www.openzaurus.org/official/unstable/188.8.131.52/feed/x11
src hentges-upgrades http://oz.hentges.net/hentges-feeds/1.0.x/stable/common-upgrades/
src hentges-collie http://oz.hentges.net/hentges-feeds/1.0.x/stable/machine-upgrades/collie/
dest root /
dest cf /media/cf/
dest sd /media/card/
dest ram /media/ram/
Move ipkg list data into RAM
lists_dir ext /var/ipkg/lists
9. Your ipkg.conf will be overwritten by ipkg-collateral during the upgrade process (someone has forgotten to set the configfile flag), so backup it NOW!
10. Now you only have to do
ipkg-link mount /media/card
and your Zaurus is uptodate.
Have fun with Hentges/OpenZaurus and your Collie!