I don't think I follow you.
I'm setting the shader channel variable through the inspector field on the Canvas, not through code.
Also, I only tested on 2017.1.0b5, since that version didn't crash like 5.6, I assume they changed something.
Ahh okay. Hopefully it'll be fixed in the next major Unity release, then?
Yep. 2017.1 should be released around June. But I think you might have to display a message if the user adds a SuperTextMesh to a canvas that doesn't have the correct shader channel.
I think I'll be able to add the shader channel myself through code when that comes, so hopefully this works! Either way, my asset that replaces Unity UI should be going up soon, too. Let me know if you want a copy!