You can simply use XAML to overlay elements on top of the SceneView.
<Grid> <esri:SceneView /> <!--Create red box at the center --> <Border Width="100" Height="100" BorderThickness="2" BorderBrush="Red" HorizontalAlignment="Center" VerticalAlignment="Center" /> </Grid>
On a side note, the .NET Runtime uses DirectX, not OpenGL to render, and there's no direct hook into that rendering pipeline.
Anything you can do in XAML you can also do in code. Sounds like you just need to get started with understanding XAML. There are plenty of WPF tutorials out there that I would suggest you go through first.
For example: Walkthrough: Getting Started with WPF