Eli has a post telling us how to export opaque data (often called custom attributes) from Max and Maya. Unfortunately those instructions seem to disagree with this post from one of the Autodesk FBX developers, which say that there is very limited support for custom attribute export. My tests show that the FBX developer is correct and that the Max exporter doesn't export string custom attribute types, which is a pity. If you are using Maya, the situation is a bit better as the comments to this Normal Map Sample mention a couple of ways of getting the Maya exporter to export opaque data.