Thanks everyone for the input, interesting discussion for sure. I decided to go with my dynamic database approach using entity and attributes. Storing metadata simply in the database while clean and flexible solution simple isn't viable since it in effect renders the metadata invisible from the standpoint of SQL processing and querying, using this system I can nevertheless export the metadata as XML that can I can perform Xpath queries against (I'd rather use Xpath than inventing a proprietary query engine) although the problem remains on how to run XPath queries efficiently against large documents or many small ones.
About the only downside to this approach is that the XML isn't freely structured but I don't see many instances where metadata need to be deeply hierarchial, seems like value-key and value-sets can handle most any cases. best regards --- Mattias Konradsson -- http://cms-list.org/ more signal, less noise.