In SwiftUI on macOS, using instancing in RealityKit, how can I set individual colours per instance?

I have written this function:

  @available(macOS 26.0, *)
  func instancing() async -> Entity {
    let entity = Entity()
    do {
      // 1. Create a CustomMaterial
      let library = offscreenRenderer.pointRenderer!.device.makeDefaultLibrary()!
      let surfaceShader = CustomMaterial.SurfaceShader(
          named: "surfaceShaderWithCustomUniforms", // This must match the function name in Metal
          in: library
      )
      let instanceCount = 10
      // No idea how to actually use this...
//      let bufferSize = instanceCount * MemoryLayout<UInt32>.stride
//
//      // Create the descriptor
//      var descriptor = LowLevelBuffer.Descriptor(capacity: bufferSize, sizeMultiple: MemoryLayout<UInt32>.stride)
//
//      // Initialize the buffer
//      let lowLevelBuffer = try LowLevelBuffer(descriptor: descriptor)
//      lowLevelBuffer.withUnsafeMutableBytes { rawBytes in
//          // Bind the raw memory to the UInt32 type
//          let pointer = rawBytes.bindMemory(to: UInt32.self)
//        pointer[1] = 0xff_0000
//        pointer[0] = 0x00_ff00
//        pointer[2] = 0x00_00ff
//        pointer[3] = 0xff_ff00
//        pointer[4] = 0xff_00ff
//        pointer[5] = 0x00_ffff
//        pointer[6] = 0xff_ffff
//        pointer[7] = 0x7f_0000
//        pointer[8] = 0x00_7f00
//        pointer[9] = 0x00_007f
//      }

      var material = try CustomMaterial(surfaceShader: surfaceShader, lightingModel: .lit)
        material.withMutableUniforms(ofType: SurfaceCustomUniforms.self, stage: .surfaceShader) { params, resources in
          params.argb = 0xff_0000
        }

      // 2. Create the ModelComponent (provides the MESH and MATERIAL)
      let mesh = MeshResource.generateSphere(radius: 0.5)
      let modelComponent = ModelComponent(mesh: mesh, materials: [material])

      // 3. Create the MeshInstancesComponent (provides the INSTANCE TRANSFORMS)

      let instanceData = try LowLevelInstanceData(instanceCount: instanceCount)

      instanceData.withMutableTransforms { transforms in
        for i in 0..<instanceCount {

          let instanceAngle = 2 * .pi * Float(i) / Float(instanceCount)
          let radialTranslation: SIMD3<Float> = [-sin(instanceAngle), cos(instanceAngle), 0] * 4

          // Position each sphere around a circle.
          let transform = Transform(
            scale: .one,
            rotation: simd_quatf(angle: instanceAngle, axis: [0, 0, 1]),
            translation: radialTranslation
          )
          transforms[i] = transform.matrix
        }
      }

      let instancesComponent = try MeshInstancesComponent(mesh: mesh, instances: instanceData)

      // 4. Attach BOTH to the same entity
      entity.components.set(modelComponent)
      entity.components.set(instancesComponent)
    } catch {
      print("Failed to create mesh instances: \(error)")
    }
    return entity
  }

and this is the corresponding Metal shader

typedef struct {
  uint32_t argb;
} SurfaceCustomUniforms;

[[stitchable]]
void surfaceShaderWithCustomUniforms(realitykit::surface_parameters params,
                                     constant SurfaceCustomUniforms &customParams)
{
  half3 color = {
    static_cast<half>((customParams.argb >> 16) & 0xff),
    static_cast<half>((customParams.argb >> 8) & 0xff),
    static_cast<half>(customParams.argb & 0xff) };
  params.surface().set_base_color(color);
}

which works well and generates 10 red spheres. While listening to the WWDC25 presentation on what's new in RealityKit I am positive to hear the presenter saying that it is possible to customise each instance using a LowLevelBuffer, but so far all my attempts have failed.

Is it possible, and if so how ?

Thanks for reading and for your help.

Kind regards, Christian