> How hard would it be to make your workbench pull and render code from GitHub repositories, so that for example, authors can link to a render of the codebase in their readme?

You can already do some of this. Either by pointing to an encoded url (something like this: https://studio.replicad.xyz/workbench?from-url=https%253A%25...).

There is also a share arrow which will put the current code in an url param (and you can have something like this: https://studio.replicad.xyz/workbench?code=UEsDBAoAAAAIANhzV...