create ISmsGpuView and have the Sms VdpViewer tool depend on it

src/BizHawk.Client.EmuHawk/MainForm.Events.cs

@@ -1550,7 +1550,7 @@ namespace BizHawk.Client.EmuHawk
 
 		private void SmsSubMenu_DropDownOpened(object sender, EventArgs e)
 		{
-			SmsVdpViewerMenuItem.Visible = Game.System != "SG";
+			SmsVdpViewerMenuItem.Enabled = Tools.IsAvailable<SmsVdpViewer>();
 		}
 
 		private void SmsBiosMenuItem_Click(object sender, EventArgs e)

src/BizHawk.Client.EmuHawk/tools/SMS/VDPViewer.cs

@@ -12,8 +12,7 @@ namespace BizHawk.Client.EmuHawk
 	public partial class SmsVdpViewer : ToolFormBase, IToolFormAutoConfig
 	{
 		[RequiredService]
-		private SMS Sms { get; set; }
-		private VDP Vdp => Sms.Vdp;
+		private ISmsGpuView Vdp { get; set; }
 
 		private int _palIndex;
 

src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/ISmsGpuView.cs

@@ -0,0 +1,14 @@
+using BizHawk.Emulation.Common;
+
+namespace BizHawk.Emulation.Cores.Sega.MasterSystem
+{
+	public interface ISmsGpuView : IEmulatorService
+	{
+		byte[] PatternBuffer { get; }
+		int FrameHeight { get; }
+		byte[] VRAM { get; }
+		int[] Palette { get; }
+
+		int CalcNameTableBase();
+	}
+}

src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/SMS.cs

@@ -208,6 +208,11 @@ namespace BizHawk.Emulation.Cores.Sega.MasterSystem
 			// stops a few SMS and GG games from crashing
 			Cpu.Regs[Cpu.SPl] = 0xF0;
 			Cpu.Regs[Cpu.SPh] = 0xDF;
+
+			if (!IsSG1000)
+			{
+				ser.Register<ISmsGpuView>(new SmsGpuView(Vdp));
+			}
 		}
 
 		public void HardReset()
@@ -417,5 +422,21 @@ namespace BizHawk.Emulation.Cores.Sega.MasterSystem
 		}
 
 		private readonly SmsSyncSettings.Regions _region;
+
+		public class SmsGpuView : ISmsGpuView
+		{
+			private readonly VDP _vdp;
+
+			public SmsGpuView(VDP vdp)
+			{
+				_vdp = vdp;
+			}
+
+			public byte[] PatternBuffer => _vdp.PatternBuffer;
+			public int FrameHeight => _vdp.FrameHeight;
+			public byte[] VRAM => _vdp.VRAM;
+			public int[] Palette => _vdp.Palette;
+			public int CalcNameTableBase() => _vdp.CalcNameTableBase();
+		}
 	}
 }