I'm not sure about the Holy Warrior (can't remember atm), but buying Settlers with faith can still be a good strat, and it can be available to all civs taking Piety (instead of just the one taking Holy Warrior). There are certain ways to implement this too, as you mentioned. It could be modded for Settler buying with faith to work similar to GP buying, where the price increases with each Settler that you buy. So it can be fixed where you can only afford 1, maybe 2 settlers with faith buying early-mid game. Plus, if you focus faith for settlers/workers, you will likely enhance your religion much later and not have faith for religious building purchases. So it adds a trade-off there. Either enhance early, or expand early with faith buying. The strat can be tweaked so that it isn't OP. Also, what's wrong if this is strong? Isn't that what Piety needs, is strong policies? I think a policy to faith buy settlers/workers would be strong, but it only works early game. After your empire is setup, an entire policy that gives faith buying for Settlers and Workers only suddenly because useless.