Like I said, it's difficult, because the EM is being provided by EEJB3, not by Seam, so I have no place to "hook in".
Now, if you used a Seam-managed EM, it would be trivial. Just register your mock EM as a Seam component instead of installing the Seam-managed EM.
Yes great thanks - when thinking about it that is the best and most clean solution. Thanks again.
/Magnus