Custom Fields Validation

I see that cnmaestro have custom fields support but there are not a way to validate thoses. For example if I use Brazil CPF number I would like a way to validate.

Would be possibble to implement? An ideia is to call a custom externali url to validate.

By example, using post methods to the url:

name, postal code, cpf


If the url returns nothing the data are ok, otherwise the url returns the error string.