What do you mean exactly by a proxy-based solution, something like CGLib or Java dynamic proxies?
If yes, I think that would be to restrictive, for example a lot of our model/bean classes are final and don't implement a particular interface.
BTW, our use case would be a rich client scenario where validation happens on the buffered values of a presentation model.
I think my personal preference would be option 2. (although my first thought about such a solution resulted in something like option 1.)
Please consider this for 2.0. Thanks!