EclipseLink doesn't yet know how to lookup the Transaction Manager on AS7. Its possible to write integration code for AS7 to improve that. There is also app server integration code in EclipseLink as well (that this could be added to).
Have you tried posting on the EclipseLink forum and ask if/when EclipseLink could switch to using the TransactionSynchronizationRegistry instead of TransactionManager. The JTA 1.1 specification mentions the standard JNDI name for the TSR java:comp/TransactionSynchronizationRegistry.
If you post to the EclipseLink forum, you could reference this forum link, so that interested parties could discuss the best way to get this working. There are other AS7 community contributors that also want to help.