You probably would need to remove the javax.persistence classes from your deployment to avoid that error.
Having said that, I don't think that Toplink knows how to look up the TransactionSynchronizationRegistry (don't think they know how to use that yet) or to lookup the transaction manager on AS7. So, you wouldn't be able to control the transactions with JTA (at least not for updating the database).
Last I heard, most of the development is occuring in the EclipseLink project. I submitted a patch for changing the transaction manager JNDI lookup string but don't really if that has been released yet. If your able to build EclipseLink from source and find other changes that are needed in EclipseLink, I'd be interested in that. I can get you the EclipseLink bug report link if your interested.