Good idea, create an issue (feature request) on github!

Hi Mark, there was an issue with the wrong BrowsableAttribute being used in the models. The PropertyGrid is set to require [System.ComponentModel.Browsable]. See the updated example source code on github.

<Window x:Class="HitTestDemo.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:h="http://helix-toolkit.org/wpf"
        Title="Hit test demo" Height="480" Width="640">
    <!-- Subscribe to MouseDown events on the Viewport -->
    <h:HelixViewport3D x:Name="viewport1" MouseDown="OnViewportMouseDown">
        <h:DefaultLights/>
        <!-- Set the back material to null to exclude hits on the inside of the geometry -->
        <h:PipeVisual3D Point1="-10,0,2" Point2="10,0,2" BackMaterial="{x:Null}" ></h:PipeVisual3D>
        <!-- The geometry is created in code behind, subscribe to the MouseDown event 
        (when these events are handled, the viewport will not receive the MouseDown event) -->
        <ModelUIElement3D x:Name="element1" MouseDown="OnVisualMouseDown"></ModelUIElement3D>
    </h:HelixViewport3D>
</Window>

namespace HitTestDemo
{
    using System.Diagnostics;
    using System.Windows;
    using System.Windows.Input;
    using System.Windows.Media.Media3D;
    using HelixToolkit.Wpf;
    public partial class Window1
    {
        public Window1()
        {
            this.InitializeComponent();
            // Create a pipe geometry and model
            var mb = new MeshBuilder(false, true);
            mb.AddPipe(new Point3D(-10, 0, 0), new Point3D(10, 0, 0), 0, 1, 50);
            var model = new GeometryModel3D(mb.ToMesh(), Materials.Green);
            // Assign the model to the ModelUIElement3D
            this.element1.Model = model;
        }
        // this is called when the user clicks on the ModelUIElement3D visual
        private void OnVisualMouseDown(object sender, MouseButtonEventArgs e)
        {
            var position = e.GetPosition(this.viewport1);
            this.CalculateHitTextureCoordinates(position);
            e.Handled = true;
        }
        // this is called when the user clicks on the 3D viewport (also outside the visual model)
        private void OnViewportMouseDown(object sender, MouseButtonEventArgs e)
        {
            var position = e.GetPosition(this.viewport1);
            this.CalculateHitTextureCoordinates(position);
            e.Handled = true;
        }
        private void CalculateHitTextureCoordinates(Point position)
        {
            // loop over all hits (sorted by distance from camera, hidden models will also be included in this sequence)
            foreach (var hit in this.viewport1.Viewport.FindHits(position))
            {
                // get the texture coordinates of the triangle that was hit
                var tc1 = hit.Mesh.TextureCoordinates[hit.RayHit.VertexIndex1];
                var tc2 = hit.Mesh.TextureCoordinates[hit.RayHit.VertexIndex2];
                var tc3 = hit.Mesh.TextureCoordinates[hit.RayHit.VertexIndex3];
                // calculate the texture coordinate of the hit point by linear interpolation
                var tc =
                    new Point(
                        (tc1.X * hit.RayHit.VertexWeight1) + (tc2.X * hit.RayHit.VertexWeight2) + (tc3.X * hit.RayHit.VertexWeight3),
                        (tc1.Y * hit.RayHit.VertexWeight1) + (tc2.Y * hit.RayHit.VertexWeight2) + (tc3.Y * hit.RayHit.VertexWeight3));
                Debug.WriteLine("Texture coordinate: " + tc);
            }
        }
    }
}