public class ProfileModelBinder : IModelBinder
{
    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.Model != null)
            throw new InvalidOperationException("Cannot update instances");

        Profile p = (Profile)controllerContext.HttpContext.Session[BaseController.profileSessionKey];
        if (p == null)
        {
            p = new Profile();
            controllerContext.HttpContext.Session[BaseController.profileSessionKey] = p;
        }
        return p;
    }
}

public ActionResult MyAction(Profile currentProfile)
{
    // do whatever..
}