You cannot export data in custom fields. From the User Manual:
Custom metadata fields created using the Manage Custom Metadata fields dialog. Custom fields are for use only in Aperture.
Therefore your best course is to use existing fields for your data. Look through the available IPTC fields and see if one is appropriate, or select one that you won't ever use and use it for your data.
They're purely for personal use? Thanks Kirby, I feel enlightened.
Kirby's answer is spot-on. The custom tags are purely for use within an Aperture context. When you are exporting single image files to be used by other applications, you can only export tags that other applications will understand - IPTC core, or EXIF, or keywords.
But if you want to export images with your custom tags assigned to be used by Aperture in other Aperture libraries it can be done: export an album or project with your images as an Aperture Library: "File > Export > Folder/Project/Album as new library"
This way you can use all your Aperture custom tags in other Aperture libraries.
Tim -- glad to help. You've likely already resolved this better, but if you haven't, this might get the ball rolling. The data you need to have _in the exported file_ is, afaict, "Is this model released? Y/N". The specifics of the release, and the actual release, don't need to be in the file -- they just need to locatable from the information in the file. For this, a keyword suffices as long as your file naming allows you to find the Image in your Aperture Library. I would keep the custom field -- they can be, as you know, quit useful -- but add a keyword (e.g.: "ReleasedYES"). Additionally, I would create a global Smart Album set to show all Images where the custom field "Model Release" is _not_ empty _and_ the keyword "ReleasedYES" is _not_ applied. Add to your regular administrative workflow examining the contents of this Smart Album and adding the keyword "ReleasedYES" to any Images in it.
Since this information is likely very important to your business, you might consider using the most visible of Aperture's metadata: Color Labels. (The catch with Color Labels is that each Image can have only one of the eight possible labels. There are few conditions that have eight discrete states. Perhaps there are levels of release?)
Of course, your implementation is sure to be better customized, etc. I just thought this might give you some ideas.
There's no direct way as has been correctly pointed out above. But there's a couple of other approaches that you could investigate depending on the importance of exporting your data. And if you have only a handful of photos using the custom fields, then it may not be worth the time.
You can filter on your custom fields, as you probably already know. And you can access the custom fields using Applescript. This would let you create a script that would populate an IPTC core field with your custom field data, saving you the data re-entry headache.
The IPTC Extension which Aperture does not support (hopefully it will in the new version) has a group of "Additional Model Info" fields. The IPTC Extension is supported by EXIFTOOL which could then be used to actually imbed your data in the image's IPTC info.
Like I said, it might not be worth your time to research these tools and develop a process. Just wanted to point out some alternatives.
Thanks for all your guidance and help it's been very helpful and I appreciate your time replying. I have about 15,000 images that need this info. Fortunately I do already include some release details in the keywords so not all is lost. The problem I have is distributing the images to clients in bulk, they want fields specifically for 'model' and 'property' release data. It looks Iike I need to use two existing core fields for this until I can get my head around something more complex.