In Apertue you can create custom metadata views the selected fields. Under Aperture there is a database that stores all metadata, form this you can manipulate metadata as tables. All metadata are embedded at file export. You can create plug-ins that lets you enter custom metadata and then sends the images to a selected service (Photobooks on demand, image gallery, client).
Notes from the first Photo Metadata Conference. This conference was held in Florence, Italy june 7, 2007. It was organized by Cepic, IPTC and IFRA. These are my private notes published to give you some idea of what was discussed. I will later write some articles from the conference and publish those on BLF.se and DigitalFotografen.se, but probably only in Swedish. If there are any demand I might make a short translated version in English. You can find presentations and more information at phmdc.org and cepic.org .