nowton on "[Plugin: Jetpack by WordPress.com] Related posts not showing on custom post type"

Could you try to reindex your site so that Jetpack will know of the new post type?

You can reindex your site by clicking Jetpack on your site’s Dashboard, and scrolling down and clicking the “Debug” link at the bottom of the page. From there you will have a “Reindex Posts” available. If you are not seeing the Related Posts show up right away with your Custom Post Types, try clearing your browser and site’s cache.

If that doesn't help, could you try to install and activate this small plugin, by going to Plugins > Add New > Upload in your dashboard:
http://i.wpne.ws/Vdoc

Once you've activated the plugin, try to relaunch the indexation again. If the Related Posts start appearing, you can deactivate and delete this small plugin.

Let me know if it helps.