funny enough this is exactly what gab/dissenter did, giving any website a comments section. it's actually a really cool concept, though obviously dissenter ended up hella toxic.
someone made a clone, zeeker, but it doesn't have a lot of users: https://chrome.google.com/webstore/detail/zeeker-on-site-dis...
A corporate website can become a graffiti wall, and dissatisfied customers can paint on it.
The companion website shows the original website and the painted wall on top of it. Even better if the browser cooperates.