Apply modifier only in holy cities?

[Imnimo]

I'm trying to set up a leader ability that applies a modifier in holy cities, but I can't seem to find any hooks in the modifiers or requirements files that reference holy cities (except for Jerusalem's MODIFIER_PLAYER_TREAT_CAPITAL_AS_HOLY_CITY, which doesn't really help me here).

Is there a way to check whether a city is a holy city via requirements, or by some other modifier? If not, how difficult would be it be to create such a requirement via Lua?