Can you not soul trap the boss of Dawnguard? Because I've soul trapped all the guild "bosses" from the main game of Skyrim.
That's because they didn't have anything special about them, only in what they were using (
AKA: CoW, the Thalmor guy had the Eye of Magnus.
The actual person didn't have anything special about him, like Miraak did, as he was the Dragonborn).
I think Miraak is different. Plus, we Defeat him, but
I'm not saying I'm correct though, as I havn't tried it myself, but I believe he's one of those "Too Powerful" NPC's.