Just had this discussion with UL (Northbrook, Il) today, regarding 'extensions' in fire rated ceiling.
Per the White Book, 'ANY' type of plate can be installed on the rated boxes, as long as the boxes are installed to mfg/UL specs.
My inquiry related to extensions on deep set boxes, 2 hr rated ceiling; answer was yes, Arlington BE-1R is OK (4" round), but, BE-1 for 1-gang device is not. Go figure??