Note: A better solution would achieve all of the following:
- Unique variants of GW buildings (e.g., the Ceilidh Hall) would replace their generic variant when playing as the respective civilization (e.g., the Celts);
- The Royal Library would only show up when playing as the Assyrians;
- Custom unique GW buildings where the generic variant doesn't feature a GW slot would show up in accessory columns for the respective players;
- Mods that add slots to generic GW buildings would not break the Culture Overview;
- Custom generic GW buildings would be inserted according to their prereq tech rank.
Maybe a highly skilled modder takes up the challenge.