Is SDM WebService using a different method to access the MDB as against the Web Interface?
SDM WebServices calls are done by the webengine. The webengine uses spel function calls to request the MDB data using slump->virtdb->sql_agent.
This helps to keep the WebService logic the same as the SDM Web interface. Direct connection to the MDB through the Webservice is not supported, irrespective of the DBMS type.