Aha, so at least someone else will be interested in what I'm doing:-)
The idea is that ALL maintenance is done with iTunes and then regular (nightly if required) rebuilds of the special database (only takes a few minutes anyway) keeps the Rio stuff in sync, so my Rio Receivers should be able to play from up-to-date lists and syncing to the RioCar ought to be a simple process, with no need for any further manipulation of the playlists etc - that's all taken care of by iTunes.
I store the iTunes music files and it's XML database file on my linux server and mount the relevant volume on the Mac (and/or PC) via NFS. The server then reads the XML file and creates all the necessary stuff to drive the Rio Receivers using apache, either the original software or tRio for the Receivers. That bit all seems to work perfectly.
I am also creating a precise set of files (fids etc) on the server that then simply need to be uploaded to the RioCar, for which I am using rsync. The idea was that this could be done either from the command line on the empeg itself, or from the Hijack menu, or from the command line on the server. Unfortunately rsync has a problem if there's no terminal present, so only the first option works at present and that currently has an issue when doing the first bulk sync, which is what I'm working on at the moment.
Are you using iTunes on the Mac or PC?