I do not believe any A340 operator powersback. I'm not sure if the A340 has 4 reversers- if only 2, although physically it could, it would take very high power. This would cause big problems. Apart from turboprops which can happily powerback, the only jets that can comfortably do so are DC9-type formats with raised power units that would not suffer such ingestion problems. The problem with the lower power units that most jets have is re-ingesting gases and debris that are blown forward by the reverse air.