Add custom validator on unmapped field, but with context of whole form submission?

You say you have unmapped field. Would it help, if you make it mapped to the Person entity? Just make a new property in the Person class with getter and setter methods, but not to the ORM, since you don't want it persisted.

If you do not want to polute your Person class, you can also make another composite class, which will hold your currently unmapped field and a Person object (you will then make it mapped). Ofcourse you will then set data_class to match the new object's namespace.

Both above solutions should work with the upper code you have there. Please let me know it it helped.