A first draft. Looks neater than with ChatGPT. Obviously.

Author: pgrawehr

src/devices/Board/RaspberryPiBoard.cs

@@ -13,6 +13,7 @@
 using System.IO;
 using System.Linq;
 using System.Text;
+using Iot.Device.Gpio;
 using UnitsNet;
 
 namespace Iot.Device.Board
@@ -925,5 +926,11 @@ public override ComponentInformation QueryComponentInformation()
             ret.Properties["PinCount"] = PinCount.ToString(CultureInfo.InvariantCulture);
             return ret;
         }
+
+        public static VirtualGpioController CreatePhysicalPinMapping(GpioController defaultController)
+        {
+            var map = 1;
+            var pinMapping = new VirtualGpioController();
+        }
     }
 }

src/devices/Board/VirtualGpioController.cs

@@ -93,7 +93,12 @@ protected override void Dispose(bool disposing)
             _pins.Clear();
         }
 
-        /// <inheritdoc />
+        /// <summary>
+        /// This operation is not supported, as the virtual controller doesn't know what to do here.
+        /// Use <see cref="VirtualGpioControllerWithDefault"/> for an alternative approach if you need to dynamically assign pins.
+        /// </summary>
+        /// <param name="pinNumber">The number of the pin</param>
+        /// <exception cref="InvalidOperationException">Always. This operation is not supported</exception>
         protected override void OpenPinCore(int pinNumber)
         {
             // Not clear what this should do

src/devices/Board/VirtualGpioControllerWithDefault.cs

@@ -0,0 +1,61 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Device.Gpio;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+#pragma warning disable CS1591 // Missing XML comment for publicly visible type or member
+
+namespace Iot.Device.Gpio
+{
+    /// <summary>
+    /// A virtual GPIO controller that serves primarily for a pin mapping on an existing controller.
+    /// Unlike <see cref="VirtualGpioController"/> it has a default controller rather than individual pins
+    /// and thus also supports <see cref="GpioController.OpenPin(int)"/>.
+    /// </summary>
+    public class VirtualGpioControllerWithDefault : VirtualGpioController
+    {
+        private readonly GpioController _defaultController;
+        private readonly Dictionary<int, int> _fromVirtualToRealMapping;
+
+        public VirtualGpioControllerWithDefault(GpioController defaultController, Dictionary<int, int> fromVirtualToRealMapping)
+        {
+            _defaultController = defaultController ?? throw new ArgumentNullException(nameof(defaultController));
+            _fromVirtualToRealMapping = fromVirtualToRealMapping ?? throw new ArgumentNullException(nameof(fromVirtualToRealMapping));
+        }
+
+        /// <summary>
+        /// Opens a pin from the default controller
+        /// </summary>
+        /// <param name="pinNumber">The pin number</param>
+        protected override void OpenPinCore(int pinNumber)
+        {
+            int realPinNumber = MapToRealPin(pinNumber);
+            if (realPinNumber == -1)
+            {
+                throw new InvalidOperationException($"Virtual Pin Number {pinNumber} is unknown");
+            }
+
+            var pin = _defaultController.OpenPin(realPinNumber);
+            if (!Add(pinNumber, pin))
+            {
+                _defaultController.ClosePin(realPinNumber);
+                throw new InvalidOperationException($"Virtual Pin Number {pinNumber} is in use already");
+            }
+        }
+
+        public int MapToRealPin(int virtualPin)
+        {
+            if (_fromVirtualToRealMapping.TryGetValue(virtualPin, out int realPin))
+            {
+                return realPin;
+            }
+
+            return -1;
+        }
+    }
+}