Yes, the script does not yet allow this kind of simply configuration yet.
You can simply change the persistence.xml configuration file according to your database though, in
https://github.com/krisv/jbpm/blob/master/jbpm-installer/db/persistence.xml
change the dialect
and in
https://github.com/krisv/jbpm/blob/master/jbpm-installer/db/testDS1-ds.xml
change the url, driver class, username and password
And make sure you put your lib with the driver class in jbpm-installer/db/driver
You'll need to rerun ant install.demo for the changes to take effect.
Any contributions to the script to make this easier are always welcome ;)
Kris