FWIW, I've done that "Shadow Within" Jago mission against Omen ten times since that was posted and it's dropped an Omen dossier twice.
I've also done one called Shadow Puppeteer (not the one against Gargos, the one against two Omens) a bunch of times and while it hasn't given me any Omen dossiers, I'm guessing it will at some point.
I've been using Captain Spinal to try and use the mask of the ancients on his mask of the ancients mission in hopes of getting that dossier, but Jago will still get Shadow Within mission even if he's not your captain, same goes for Omen and Shadow Puppeteer. So I use those two as my second and third to try and farm Omen dossiers.
I'm currently at 97% and these are all I need besides Kilgore's.