The Book of Judgement features the "Slate Agent". Those can be fitted with a new persona and (most importantly) a fine set of knowledges to impersonate somebody else. By RAW, up to three Lores or Trades of Choice. Including Forbidden Lores. Jub, you got it. Oh, and you get +10 on the relevant skill tests for these Lores/Trade. For free. Jub, you got it.
This rank is available from 6+ onwards. Restriction: You need to be Unremarkable. As an example
FL(Cults)+10 become available for an Adept early on (Rank 3); Sisters and Arbitrators and Psykers get it in the End Game (7+). And have to pay for it.
FL(Daemonology)+10 is rather endgame... if available at all. Unless you are from the Radicals Handbook...
And so on...
My personal opinion: This is one touch to much. I will house-rule this away to "Common Lores and Scholastic Lores only" or to "Forbidden Lores only as Basic".
Other thoughts?
EDIT: Of course being an Adept beats being a Slate Agent. But otherwise, I would not see why not getting somebody to be one. Yes, is sacrifices rank. But all those Forbidden Knowledges at the tip of your hand...
