We do support validation on Workflow event in a NPD project. if you have installed EP pack, go to \ReferenceImplementations\Validation\Documentation\Validation Objects.xls, you will see a detailed listing of events,type IDs and so on.
I'm not sure what validation do you want to implement, here i have an example you can refer to:
1, add the following configuration to Config\Extensions\ValidationSettings.xml:
<!-- Example NPD Project Workflow validation requires property Brands -->
<if type="ReflectiveRequiredValidator" property="Brands" />
2, restart iis
3, go to a NPD project, clear the data in Brands field, save the project ,no error
4, workflow this project, error 'Property Brands is required.'
Hope this helps.