I'm wanting some hooks into FBX data that I know exists, yet which is not typically exposed to Unity. For instance, the FBX format supports constraints and blend shapes. If I want to get data related to these objects, do I have any other options apart from adding string userdata to each object that utilizes these data so that I can use AssetPostprocessor.OnPostprocessGameObjectWithUserProperties()?
No, there is no way to extract that info. The only workaround is to pack your special info into something that Unity supports (geometry, animations, and so on), but for thing like blendshapes it would be quite complicated thing to do.
For anyone interested, I am currently creating workarounds for these problems for use with Maya. They will be released sometime this year, but for anyone interested in e.g. replicating the behavior in another application, here are some examples:
Getting constraints: http://adammechtley.com/2010/04/coming-soon-complete-maya-unity-workflow/
Getting blend shapes: http://adammechtley.com/2011/01/gdc-and-a-new-years-resolution/