Brian is working on https://bugs.eclipse.org/258544 which is possibly
going to work for your situation. He is adding support for referencing
jars in the persistence.xml. I am not completely sure if this will cover
your use case, but it will certainly go a long way towards supporting it.
That bug will be completed for Galileo. Currently we do not support your
use case, so your workaround is the best we can do for now :)