See details here:
http://sitecoreblog.alexshyba.com/2012/07/pre-production-preview-ultimate.html

To build, create "sc.lib" folder under the project root and drop Sitecore.Kernel.dll in there.

For this to run, the following assets will need to be deployed:
- the assembly into the /bin
- the .config file into App_Config/Include

Configration:
- Depending on your Workflow configuration, the "PreProdPreview.FilteredWorkflowStates" setting will need to contain the list of workflow states that you would like the filtering logic to respect