首先可以确定的是，过去将来完成进行时是有被动语态的，表示在过去时间段里认为某事有可能在将来时间段被人正在完成。比如说，He said the trees would have been being planted on March 12th on the mountain this year by the students,because the school had already decided.

但是像这样的句子在平常口语中是不常用到的，所以只需要了解一下就行了。