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.
--Kirby.