Multiple databases in one application with locations unknown

Hello everyone,
I am developing an application that uses several databases, one database for each user. The users, and the parameters for their database connections, are read from a configuration-file at runtime.
I would like to use JPA to access the databases. This is where the problem arises, because if I try to get an EntityManagerFactory, I have to specify a name; I just use <username>-pu. This causes a PersistenceException because "No Persistence provider for EntityManager named maarten-pu". Which is correct, because my persistence.xml contains no persistence units.
I cannot define all persistence units beforehand, because at that time the configuration-file is not yet read.
Does anyone know how to solve this problem?
Kind regards, and thanks in advance,
Maarten

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