ActionScript 3.0 :: Form Validation In Flash?

I'm working on a form validation script, and whilst I like (in theory) to build reusable classes, this is one time I'm finding it a bit tricky.

The problem is it's difficult to build something loosely-coupled when the actual components of the form could be textfields, Flash components, or even my own basic custom components.

Interfaces are out the window, as an interface (say IFormElement) I might set up with getters and setters for label, error message and value, won't be relevant to Flash UI components.

At the moment, I'm leaning towards a base Validation class that should be extended in order to cater for different flavours of forms, with getValue() and setError() type messages.

The problem again here is that it is difficult to retain type safety, as when overriding the methods in the subclasses, obviously one can't change the type within the method signature.