An example mod. Chrome doesn't restrict the set of tabs any more, though i'm not sure what happens if you make a new tab :D

OpenRa.Game/Chrome.cs

@@ -47,7 +47,6 @@ namespace OpenRa.Game
 		// Build Palette tabs
 		string currentTab = "Building";
 		bool paletteOpen = false;
-		static string[] groups = new string[] { "Building", "Infantry", "Vehicle", "Plane", "Ship" };
 		readonly Dictionary<string, string[]> tabImageNames;
 		readonly Dictionary<string, Sprite> tabSprites;
 		
@@ -111,7 +110,6 @@ namespace OpenRa.Game
 			optionsBackground = SpriteSheetBuilder.LoadAllSprites("dd-bkgnd")[Game.CosmeticRandom.Next(4)];
 			
 			tabSprites = Rules.NewUnitInfo.Values
-				.Where(x => groups.Contains(x.Category))
 				.Where(u => u.Traits.Contains<BuildableInfo>())
 				.ToDictionary(
 					u => u.Name,
@@ -121,6 +119,8 @@ namespace OpenRa.Game
 				.ToDictionary(
 					u => u.Key,
 					u => SpriteSheetBuilder.LoadAllSprites(u.Value.Image)[0]);
+
+			var groups = Rules.NewUnitInfo.Values.Select( x => x.Category ).Distinct().ToList();
 			
 			tabImageNames = groups.Select(
 				(g, i) => Pair.New(g,
@@ -644,27 +644,27 @@ namespace OpenRa.Game
 
 		void StartProduction( string item )
 		{
-			var group = Rules.NewUnitInfo[item].Category;
+			var unit = Rules.NewUnitInfo[item];
-			Sound.Play((group == "Building") ? "abldgin1.aud" : "train1.aud");
+			Sound.Play(unit.Traits.Contains<BuildingInfo>() ? "abldgin1.aud" : "train1.aud");
 			Game.controller.AddOrder(Order.StartProduction(Game.LocalPlayer, item));
 		}
 
 		void HandleBuildPalette(string item, bool isLmb)
 		{
 			var player = Game.LocalPlayer;
-			var group = Rules.NewUnitInfo[item].Category;
+			var unit = Rules.NewUnitInfo[item];
 			var queue = player.PlayerActor.traits.Get<Traits.ProductionQueue>();
-			var producing = queue.AllItems(group).FirstOrDefault( a => a.Item == item );
+			var producing = queue.AllItems(unit.Category).FirstOrDefault( a => a.Item == item );
 
 			Sound.Play("ramenu1.aud");
 
 			if (isLmb)
 			{
-				if (producing != null && producing == queue.CurrentItem(group))
+				if (producing != null && producing == queue.CurrentItem(unit.Category))
 				{
 					if (producing.Done)
 					{
-						if (group == "Building")
+						if (unit.Traits.Contains<BuildingInfo>())
 							Game.controller.orderGenerator = new PlaceBuildingOrderGenerator(player.PlayerActor, item);
 						return;
 					}

OpenRa.Game/GameRules/Rules.cs

@@ -75,6 +75,8 @@ namespace OpenRa.Game
 			if( useAftermath )
 				yamlRules = MiniYaml.Merge( MiniYaml.FromFile( "aftermath.yaml" ), yamlRules );
 
+			yamlRules = MiniYaml.Merge( MiniYaml.FromFile( "[mod]Separate buildqueue for defense.yaml" ), yamlRules );
+
 			NewUnitInfo = new Dictionary<string, NewUnitInfo>();
 			foreach( var kv in yamlRules )
 				NewUnitInfo.Add(kv.Key.ToLowerInvariant(), new NewUnitInfo(kv.Key.ToLowerInvariant(), kv.Value, yamlRules));

OpenRa.Game/Traits/ProductionQueue.cs

@@ -33,8 +33,8 @@ namespace OpenRa.Game.Traits
 			{
 			case "StartProduction":
 				{
-					string group = Rules.NewUnitInfo[ order.TargetString ].Category;
+					var unit = Rules.NewUnitInfo[ order.TargetString ];
-					var ui = Rules.NewUnitInfo[ order.TargetString ].Traits.Get<BuildableInfo>();
+					var ui = unit.Traits.Get<BuildableInfo>();
 					var time = ui.Cost
 						* Rules.General.BuildSpeed						/* todo: country-specific build speed bonus */
 						 * ( 25 * 60 ) /* frames per min */				/* todo: build acceleration, if we do that */
@@ -42,17 +42,17 @@ namespace OpenRa.Game.Traits
 
 					time = .08f * time;						/* temporary hax so we can build stuff fast for test */
 
-					if( !Rules.TechTree.BuildableItems( order.Player, group ).Contains( order.TargetString ) )
+					if( !Rules.TechTree.BuildableItems( order.Player, unit.Category ).Contains( order.TargetString ) )
 						return;	/* you can't build that!! */
 
 					bool hasPlayedSound = false;
 
-					BeginProduction( group,
+					BeginProduction( unit.Category,
 						new ProductionItem( order.TargetString, (int)time, ui.Cost,
 							() => Game.world.AddFrameEndTask(
 								_ =>
 								{
-									var isBuilding = group == "Building";
+									var isBuilding = unit.Traits.Contains<BuildingInfo>();
 									if( !hasPlayedSound && order.Player == Game.LocalPlayer )
 									{
 										Sound.Play( isBuilding ? "conscmp1.aud" : "unitrdy1.aud" );

[mod]Separate buildqueue for defense.yaml

@@ -0,0 +1,23 @@
+IRON:
+	Category: Defense
+PDOX:
+	Category: Defense
+PBOX:
+	Category: Defense
+HBOX:
+	Category: Defense
+TSLA:
+	Category: Defense
+GUN:
+	Category: Defense
+AGUN:
+	Category: Defense
+FTUR:
+	Category: Defense
+GAP:
+	Category: Defense
+SAM:
+	Category: Defense
+MSLO:
+	Category: Defense
+