Actually, you were nearly there with WHERE and ClassName. I was able to set up an example. This is what you need:
- Uni selector control
- Object type: CMS.Document
- Return column name: NodeGuid
- Display name format: {%DocumentName%}
- Selection mode: Multiple
- Additional columns: ClassName
- Where condition: ClassName in ('DancingGoatMvc.Article', 'DancingGoatMvc.Cafe')
Also, I'd recommend storing NodeGuid instead of NodeId as Kentico doesn't maintain IDs if you move items from one environment to another. If you store IDs and export-import pages (or use Staging application) you may end up with wrong pages selected.