The carrot we could offer is forgiving/forgetting, his prior crimes. He cooperates and we take down half-face, his misdeeds are forgotten. Or, wife/kids off to penal colony, and he gets a hot date with Ol' Sparky.
Using our Credit Guild covers could work, though we'd have to be surreptitious about it. No striding about in golden robes. Probably just flashing ID's and saying we're following up some irregulates in the wake of Lord Drimmle's death, and Kant's name has arisen with concerning regularity, and checking some other sources and collaborating with internal affairs has produced a disturbing accusation well-furnished with evidence: that Kant has been helping Boss Half-Face smuggle dangerous weapons into the Mid-hive.
Think that'd work?