As a follow-up to my earlier posts, EASA today approved the battery modifications so European operators are now cleared to return the 787 to service (or introduce it into service) with the modification in place.
From EASA's home page:
The Agency today approved the design change of the Boeing 787-8 Auxiliary Power Unit battery, battery charger and battery enclosure installation and certifies that the changes proposed by Boeing meet the EASA Type Certification Basis requirements. The applicable airworthiness directive will now be updated which will allow the European operated aircraft to return to service as soon as the modification will be installed.