Had help from cybereality on how to do this.
If you want to see a brief overview video:
Video outlines the features of the Wireframe shader I made. It took about 7 days.
Source code and test scene here: https://github.com/jasonswearingen/godot-csharp-tech/tree/dev/demo-projects/scratch/demos/WireframeMeshDemo but you will probably want to copy/paste to your own project.
Adding wireframes takes to parts: pre-processing your mesh with the MeshDataTool and then a shader that can read the data it encodes into the vertex color.
The MeshDataTool processing is done in WireframeMeshDemo.cs and I did my best to comment the code. I ended up using 5 color channels for vertex ID's, and 1 channel to hint what side is longest (for interrior edge removal). I was able to have these 6 channels by encoding 5 choices into red (20% each) and 1 into green. The shader reads this encoded data and generates the barycentric coords for the rest of the shader to use. The shader is in WireframeMeshDemo.shader
If you have any questions, just ask.
