Hi Kestrel thanks for your post! Sorry about the trouble uploading the image! I was able to add it successfully and I've included it below so that others can see what you're describing.
Unfortunately it's not possible to configure custom shapes so that newly created lines will automatically include "yes" and "no" text. However I definitely see how that functionality would be useful beyond the decision diamond shape. We so appreciate your feedback and if you're willing to share we'd love to learn more about your use case and what you'd like to see in this experience.
As a workaround you could consider add short line pieces to your custom shape with the words "yes" and "no" included. From there you could click and drag to extend the lines as you normally would!