Remember that this use case is very rare.
You are talking about a class level constraint associating a custom constraint violation on a specific class level element contained in a list accessible by a property "persons" from the class being validated.

I don't things we need to optimize this use case. I see addBeanNode as more useful in the more common case where you have a property level constraint on a property returning List. Say the constraint is on the "persons" property, the custom constraint violation would be declared with

constraintrValidatorContext 
    .buildConstraintViolationWithTemplate("constraints.CompatiblePersons.gender")
        .addBeanNode().inIterable().atIndex(0) 
        .addConstraintViolation();