The main problem with spoiler userscripts for chat is that generally, they require you have the userscript installed in order not to get spoiled.
Using cutting-edge Unicode shenanigans, it's possible to insert arbitrary data in a string, which makes this new userscript possible:
Introducing the chat spoilers userscript!
Using this script adds a new button near your "send" button:
Click this button to send your already-typed message as a spoiler that, critically, won't be visible to people that have not installed the script.
The messages sent that way look like this to commoners:
the link, of course, points to the install page
but people that have the userscript get this instead:
clicking the button reveals the message:
