Agreed that Mauer is a logical "interested in exploring magic beyond the strictures of the Colleges" character. At a minimum, I have made him Conspiracy participant to that extent (also because he finds the rush to judgment style and authority granted Sigmarite order grating - I give the Conspiracy a veneer of "seeking Verenan justice"). The Unity is a good fit, though I don't know if they would trust him in my version - I've made him regarded as being a bit loopy, a known "skaven believer" which makes him sort of the X-Files Mulder type figure in the Colleges (oh Mauer, seeing whiskers and cheese behind every shadowy presence) - in Conspiracy terms one of his frustrations with the Empire's status quo is its refusal to see the clear and present danger lurking beneath every city.
I've made the Black Cowl's "criminal empire" also a Bootstrappers Guild organization. Though the Space 2 benefit is the Intimidating Presence (gain fortune die on Intimidation), because when not terrorizing people into joining the 'man in the black cowl' actually promises advancement, the chance to rise up out of the brass tier and reinvent oneself etc. Apart from fluff, this fits in with the Conspiracy's general attitudes of "letting quality rise and not be constrained by the dead hand of the Imperial social structure, Sigmar's fanaticism and the constraining rules Teclis set down for those he regarded as inferiors".
The Young Blades dueling society of Nuln is also involved, the noble Pistolier in my campaign is part of it (resigned commission to avoid a duel related scandal explaining him being at loose ends). I intend them to have a Slaaneshi spin and link to later appearance of Jade Sceptre
The Sun Society is really the "outer layer" of another group. Interested in "true history" (Erhwig von Dankien "Chariots of the Frogs" stuff, though I don't see the true old ones as Slaanish at all, more Yithian [which is to say body jumpers without true form anymore]). Getting evidence of this (the gold plaque inscription etc.) was actually the real interest in the Southlands Expedition.