It could easily be directly tied to thieving or bardic skills, pickpocketing, some kind of performing skill and disquise could be requirements for a person to gain the skill, after which, different lvls could have further requirements, such as a high charisma(or some equivelent) in order to gain access to the guild chat channel.
and as far as the master spy thing, the only restriction I would put in would be that they could only infiltrate one guild at any given time, having to drop one in order to infiltrate another
heh, It could even be possible for a person to disquise him/herself as an npc, giving a \"quest\" to pcs, when the pc performs his task, bringing the spy a valuable item, they receive some bauble that\'s hardly worth going after... all that good stuff, hehe, it would surely put a damper on those that would write up a website giving answers to all the quests... find out that the npc they just got a trinket from isn\'t there the next day