After reading @arrjayjee's thread I spent an hour or two writing a simple userscript emulating the shortcut behaviour as used by RES on reddit.

This script will add a button next to "subscribe" in a subverse page, labeled "+ shortcut" or "- shortcut" depending on whether the particular subverse is already in the shortcut list or not.

Here is the link to the userscript (updated).

This is the first time in 5 years that I use javascript so it is very probably not very good code.

If this is not the right place to post this, I'm sorry and I'll delete it promptly.