Note: just to underline that there are no marketing purposes with the following link, please find a service of your convenience; it's just one of the several available and my personal choice.
In order not to lose anything (like in this case where it was deleted), I used Zapier to create a trigger that sends new posts from Reddit to Gmail.
Reddit Enhancement Suite adds buttons to collapse posts, if you're able to install it: https://redditenhancementsuite.com/
Not sure if that solution you linked to would work - it requires wrapping the post in a <details> tag as far as I can tell, which I don't think Reddit would allow. You could maybe do it with CSS using the :active state or something but I don't know if you can limit that to particular posts, as you said.
Edit: As /u/Skrioman says above this might actually be in vanilla Reddit.
It's a automation web service thing, stands for "If this, then that". Basically links various things together so an action in one triggers and action in another.
E.g. "If I favorite a tweet on Twitter, save it to Evernote". Someone made one so that if the author posts a comment, it sends a push notification to your phone.
Here is the main site: https://ifttt.com
And here's a link to the recipe by /u/takuhi:
https://ifttt.com/recipes/416995-get-a-notification-when-9h9m9e9-makes-a-new-comment
It seems to have also been anthologised in "A Blink of the Screen", a TP short story collection according to wikipedia | wordcat info
Not a story itself but there is a great book by Frank Rose about immersive storytelling which pointed me to some really interesting things, especially those making fiction using non-traditional media.
Also of course all the related writers who built on his mythos and expanded it.
Also the clive barker short story collections and of course 'the great and secret show'