We thought we were very original when we thought of the validations for plain. It turns out that there is a NHibernate Validation project that uses the same idea.
Although we were not aware of the NHIbernate Validation (and I guess the other way around) the way the validations are defined are very similar. The biggest difference is that Plain validation is focused on the domain model and the UI. NHibernate is focused on the domain model and the data layer. Plain Validation has (or it is supposed to have) translations for displaying validation errors directly in the UI.
As the two validation frameworks have very similar capabilities, we might make a mapping for you.