This is too easy... there is no need for a hack really.
1) Go to your Admin CP.
2) Go to "Replacements" and click on "Modify"
3) Under "Default" click on "Add Custom Replacement"
4) On the next screen, enter:
Code to find: [line]
Code to insert: <hr>
That's it!
Each time you put [line] in a post, it will be replaced by a <hr> line.
Works on my forums.
You may have to install this hack to make this work - it's a bug fix for 2.2.3 and up that allows replacement codes to work in posts:
https://vborg.vbsupport.ru/showthrea...threadid=35894
You can then make a button for it to insert [line] wherever you want.