I\'m currently using the ModelStateDictionary in asp.net mvc to hold validation errors and pass then back to the user. Being able to check if the whole model is valid with Model
Why not simply add a list of warnings, or a dictionary, to the ViewData and then display them in your view?
e.g.
ViewData[ "warnings" ] = new[] { "You need to snarfle your aardvark" } ;