Looks useful! Could you provide an example of a field with validations depending on the value (or validation status) of another field?
E.g., birth_month required if birth_year is set and after a certain year.
E.g., birth_month required if birth_year is set and after a certain year.
You could also write a custom check that used the previous result as documented here: http://www.fieldval.com/docs/fieldval/Check%20Functions