It seems like this should be do-able - I've logged a bug for it :
I haven't tried it, but it seems like the persistence.xml file allows you to specify a hibernate.ejb.cfgfile (your hibernate.cfg.xml) - see http://bill.burkecentral.com/2007/07/06/co-existence-with-hibernate-jpa-and-ejb3/ - but it seems like you may still have specify some mapping information in your hibernate.cfg.xml. Does that come any closer to what you are looking for?
I did see the option of specifying the hibernate.cfg.xml in the persistence.xml and thought this could be an option.
My only concern/question is related to mappings. Will I have to create .hbm files for all of my persistent entities (even though the mappings are already in annotations)? I have not seen this documented anywhere and would be a maintenance issue in the long run (have to edit the .hbms whenever we change a mapping annotation).
Agree with the feature request, btw.
Thanks for the help.