I believe that this is something to do with writing the IFC Header file - the objects that can be written out from TBC have to have originated from an IFC file - at least that is the current situation - so e.g. if we import Solids from a DWG file, we create them as IFC Shell Objects, but we cannot export them to an IFC file because they did not originate from an IFC file.
The objective of the IFC Exporter (created by the Geospatial team) was that you can import an IFC file, move, rotate and scale it into the correct position and then export it again to an IFC for use in Access. I believe that what happens on Export is that the same IFC file as was imported is written out with the Rotations, Translations and Scales as a parameter set that Access uses to move the IFC around internally. This means that the IFC objects in TBC cannot be randomly selected from different files and combined on output - at least not today. You can subset a file - but I don't believe that you can combine data from different files - at least not today.
Alan