The evolution parent rule doesn't apply to all Ghostly dragons because the only Ghostly dragons that are "evolved" are the Primary ones (Fire, Cold, Plant, and Earth) and if you have the effigy you can simply evolve a regular primary to get one. I've managed to breed Ghostly Flower (and Poison) from Ghostly Fire and Ghostly Plant. For the limited regular dragons that have a Ghostly counterpart (Ghost, Spirit, Nosferatu, Trinket, etc) you can still breed the Ghostly version using the right elements in their Ghostly form.
But Smiley is right in saying that you can only breed a Ghostly Primary if both parents are the same because they are evolved from their regular counterpart. It's different if you have the effigy. Just like if you have all the decorations required to evolve a dragon, you still can even though they are technically expired. It will cost you gems though (I believe 1,000 per evolution).