configuring the database connection in JPA persistence.xml is always ever a pain. Either you add you maintain multiple persistence.xml or you use JNDI for getting the underlying database connection for different Servers and ProjectStages.
The solution is to introduce an own DataSource which can be configured in persistence.xml. This 'ConfigurableDataSource' knows itself where to get the settings from. In our case this will get configured via standard CDI mechanisms, e.g. @ProjectStageActivated and @ExpressionActivated.