I reached out to the Slack folks with this request and got the following reply.
We have private Slack and Discourse instances and I’m looking for ways to better integrate the two. We also use Google Apps extensively.
It’d be awesome if there were a way to configure Slack with credentials to retrieve content from these other applications and unfurl/expand them accordingly.
I searched and haven’t found any way to do this yet. Is it already possible? If not, is this a request that could be considered in the future?
Thanks for taking the time to write in! A better standard for custom unfurling of private content is something we’re indeed looking to offer further down the road.
For now, we’d recommend you start by creating a Slack app, since we already have features that will get you close to what you’re looking for:
OAuth would allow teams who’ve added your app to send an authenticated request from Slack to your service (triggered by the posting of your private links).
You can use a outgoing webhook or bot to listen for these links, retrieve the relevant content, then send it back in a payload to the team as a workaround to offer this content.
I hope this suggestion helps you get started, and please let us know if there’s anything else we can help with!