IMO I believe that you could keep the DefaultDS using Hypersonic in a production environment.
Since JBoss 6 only use it to it's internal data there will be only one process touching it so you will not encounter that weekness of Hypersonic.
JBoss also creates the needed tables as memory tables, thereby getting a higher thruput compared to an external datasource.
All your deployed applications should, however, use appropriate external datasources.