At long last I've managed to sync my 5600 with Notes.....
As mentioned by another ZUG member, Intellisync for Palm 5.00.02 has a connector for Notes 5 and reported success using a combination of that and the latest 5600 Zaurus sync software (which uses a rebadged version of Intellisync).
The procedure is simple- uninstall Zaurus software and any other versions of intellisync. No need to clear the registry. Simply install Intellisync 5.00.02, install the Z software (choose Intellisync and not Qtopia Desktop), reboot and voila- in the Z sync options you should have connectors for Notes 4 and 5.
You may be able to retrofit the Notes conector DLLs into an existing Zaurus installation but I didn't try that.
Incidentally if you are using Notes 6.x, you can install a 5.x Notes client on your PC alongside the 6.x client. Configure it as normal and Intellisync will be able to hook into it to access your mailbox/calendar. The 5.x client loses some calendar functionality when accessing a 6.x mailbox, but nothing that should get in the way of syncing entries.
I did get an error message about API initialisation at the end of the sync process, but everything was synced sucessfully afterwards.
It might seem a simple thing, but now I feel my 5600 is an actual PIM rather than just a funky linux device. Happiness!