Well, I didn't think this discussion could be so interesting.

In any of your approaches, I find difficult to put the condition "there_is_a_spy_of_this_faction_in_a_settlement_with_large_stone_wall_building_level". Because only the conditions BuildingPresent and/or EndedInSettlement are not enough for my purpose.
Any suggestion about this?

About the condition FactionwideAncillaryExists, is it implemented in RTW or must we wait for BI?
My idea was also to build this invisible building for free, as it is only a technology acquired by spying, given that shopping seems to be unfeasible.
The other possibility is to gain this ancillary if a settlement with large_stone_wall is conquered by a barbarian faction.

Regarding the last point of Dromikaites: if the ancillary is given to all the characters and agents, could it be only checked in the FactionLeader? As there is only one, this method would eliminate any problem of double check due to names.