>>UPDATE. Found this http://stackoverflow.com/questions/872796/asp-net-mvc-accessing-view-model-from-a-custom-action-filter which may be a way to go.
>
>I've only used the approach with WebAPI rather than the MVC model but they are substantially the same. Looks like that might work.

We're also using API Controller and the code from that link is not working.

Here is what I am trying and can not figure out how to get to the model:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;
using Glimpse.Core.Tab.Assist;

namespace SysManager.Web.Infrastructure.Filters.Api
{
    public class AuditActionFilter : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext filterContext)
        {

            var model = filterContext.ControllerContext.Controller; // ViewData.Model is not working here
            base.OnActionExecuting(filterContext);
        }       
    }
}