I also am not an expert on accessing databases via the classpath subprotocol. However, the Developer's Guide says this in the section titled "Accessing databases from the classpath":
"In most cases, you access databases from the file system. However, it is also possible to access databases from the classpath. The databases can be archived into a jar or zip file or left as is.
All such databases are read-only.
To access an unarchived database from the classpath, specify the name of the database relative to the directory in the classpath. You can use the classpath subprotocol if such a database is ambiguous within the directory system."