Oh, that's a cool idea.
I imagine a variant would be for an Archon to do something similar, except spread the shadowfield-equipped guards around, and give each of them their own bodyguards (possibly voice-modulators or such as well). In essence, you'd never be sure whether you were speaking to the Archon, or just a stand-in cloaked in shadows.
On a mechanical front, I wish shadowfields were something like '4++ and enemies have -1 to hit the bearer'. Because then they probably could be an upgrade for Incubi without being too strong (or just unfun). As you say, it would be a lot of fun for a themed army.