If I did add it, it might be an option for the Shadowseer to be upgraded, like the Death Jester gains his Veteran Status. It would maybe upgrade his stats in a similar fashion, and give him the Master Infiltrator rule...? I don't know - I'll think about it.
Yeah, they aren't great at anti-horde. But then, everyone has to have a weakness of some sort, don't they? Against Orks, I'd suggest the bigger guns on the DJ's (Brightlance, Dark Lance, etc) to pop their vehicles. Of course, if you are facing a horde like this, that doesn't help. In that case, if you know in advance, I'd say go for things like Shrieker Cannon or Splinter Cannon and run them with the Harlequin Troupes. Their Assault Weapon status would keep them firing on the move, and the Troupe would provide more protection for them. Powerblades are a nice addition here.