I can confirm the behavior you are seeing. If you set the data source to the floor plan drawing, so that changes to the data there are reflected in tag, then editing of the data is disabled in another construct (in PN) or another file (outside PN) that externally references the floor plan file.
Outside or inside of PN, I was able to select the external reference, right click and choose Edit Xref in-place..., and then edit the data attached to the spaces. Inside PN, it would be easy enough to open the floor plan construct.
One thing I did find is that if I selected the tag added to the reflected ceiling construct (with data source set to the floor plan construct file) and tried to edit the room name on the Extended Data tab of the Properties palette - which was not grayed out - I would get an alert stating that the file was locked, and then the floor plan file would be locked - from Refedit, from opening through PN, from opening directly - even though no DWL file was generated. The only way I could clear the lock was to shut down ADT 2006 and restart it. Very annoying.
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David Koch
Autodesk Discussion Group Facilitator
Using ADT 2004 at work; access to 2005, 2006 & 2007 at home