What is the ruling on a husband anally penetrating his wife if that is her desire, and is it permissible to use an artificial phallus to satisfy her desire?

It is forbidden to approach a woman in her anus, and the one who does so is cursed by the Messenger of Allah, peace and blessings be upon him. What you mentioned about your wife - if it is proven about her - indicates that her innate disposition (fitra) is inverted. Therefore, it is not permissible for you to fulfill her desire in this matter, even in the manner you described, as this is a pretext for intercourse (in that way), and blocking pretexts is obligatory.