It would require full compatibility to be useful. I suppose most users of GitHub Pages don't actually use customization. Gojekyll could be used for building these, otherwise fall back to standard Jekyll.

I believe GitHub Wikis are also rendered with Jekyll? Without customization capabilities on the user side, Gojekyll could be used here, too.

> I suppose most users of GitHub Pages don't actually use customization

I don’t think you can actually load custom plugins, at least that wasn't the case the last time I looked.

> I believe GitHub Wikis are also rendered with Jekyll?

I don't see a reason to, they're just flat markdown documents, not a blog.