I realise that the subject of this discussion is more widespread than Decree of Khirad alone, and I've since then found similar powers (Unhinging Strike and subsequent powers) and discussions about this subject. I've also noted a lack of consensus about this issue. Now, to add to the debate...
If you can force a creature to make an attack against a creature, can this attack be against itself? The power in question is Decree of Khirad.
I've searched the DDI Compendium, but I couldn't find an entry on the rules of targeting a power there.