Scheduling elements in linked files, and the Filter by Level issue.

We are using a door schedule which filters doors by Level. We have several linked files, which have some doors as well. Once you click in the "Include elements in linked files", in the Fields Tab of the Schedule Properties, the Level parameter disappears from the list of available parameters in the Filter Tab.... This means that you cannot filter by Level if you are trying to schedule elements in linked files.
Most likely, there is a way to use some other parameter by which you can filter your doors, but this introduces a new item in the list of human-driven errors that your project will have.

The reason why Revit behaves in this way (at least in Revit 2009), quite obviously, is that Levels in linked models may not match Levels in the host model, even if they are called in the same way. In other words, two models with the same identical set of levels, may still be nested in a way that make the levels not match.
Still, one would have hoped that Revit would allow you to "calibrate", or "register", the levels of nested models, so that scheduling elements by Level would be possible. In a similar way in which you can map phases, actually.

I will check on 2010 and 2011, but if someone has some good workaround, or suggestions on this topic, please, feel free to comment...