We actually do have multiple content types which inherit from one common type. I think the architectural problem is unavoidable. Creating a separate site column for each derived content type, containng the sub-types of that content type, is what we are trying to avoid, becuase it gives us an extra column for each content type in the library. this is the way we have always done it in the past. I was hoping we could use a filter on a common list to avoid it.