I've been emailing with someone who succeeded in converting a Bookcase file into a Pilot-DB database file. He used the JPilot-DB XML importer. There is a bit of a problem with boolean fields, as evidently, the database sees an empty element as having an empty value, rather than a true boolean value. But if you run the Bookcase file through a short XSL transform to turn <read/> into <read>true</read>, I think it fixes that problem.
If I have the time to check through the JPilot-DB library, maybe I can eventually add that as an export option in Bookcase.