One might try using the JSS:NEW operator which has a slightly more aggressive method of locating Java classes with the [java.sql.DriverManager API][1], which is the preferred way of loading JDBC drivers rather than the Class.forName() method.
[1]: http://docs.oracle.com/javase/6/docs/api/java/sql/DriverManager.html
Even this didn't work in my case. (java:jnew "org.postgresql.Driver") #<org.postgresql.Driver org.postgresql.Driver@48507269 {2B42FB92}> (java:jstatic "registerDriver" "java.sql.DriverManager" (java:jnew "org.postgresql.Driver")) NIL (open-database) ; Evaluation aborted on #<JAVA-EXCEPTION com.hp.hpl.jena.db.RDFRDBException: Failure to instantiate DB Driver:PostgreSQL java.sql.SQLException: No suitable driver found for jdbc:postgresql:jena_test {6E18E546}>.