Yes, you would have to write a separate custom call.
Although this means you wouldn't get as much use from the FluentValidation validators, you need to evaluate where FluentValidation is actually adding benefit, or where trying to make it fit with your rule just adds additional complexity (the same is true for any framework/library). Personally, if I have a rule that I'm finding too complex to implement in FluentValidation then I have no qualms in bypassing FV and writing the validation logic manually...use whatever is simpler in your particular scenario.
Jeremy
Although this means you wouldn't get as much use from the FluentValidation validators, you need to evaluate where FluentValidation is actually adding benefit, or where trying to make it fit with your rule just adds additional complexity (the same is true for any framework/library). Personally, if I have a rule that I'm finding too complex to implement in FluentValidation then I have no qualms in bypassing FV and writing the validation logic manually...use whatever is simpler in your particular scenario.
Jeremy