Just adding a comment box should be pretty easy since @fiatjaf already did all the hard parts with nocomment, wire that into a WordPress plug-in and it’s done. 

But doing just this would not be a benefit for the website author, it would lead to spam and WordPress also already supports comments itself, with moderation and anti spam plugins (that are not compatible with nostr).

The only way I could see it working is if there was some way to apply a WoT filter from the WordPress side. 

Then the benefit would be that it would attract website activity from nostr users. While small now it could be big in the future and we have to start somewhere. The other way around it could grow nostr users when people see a conversation on a website and want to reply, giving them a reason to get on nostr.

Under the hood it would essentially be a nostr conversation around an URL, something similar like I described here: nostr:note1gkm2wmgc8zumke0cefpzsrxafn7rmsmqhdy9stw5p0wqjz22kgfqv2jgmf

People would be replying to an article or page on nostr, could be within a nostr app or on the website.  WordPress would just show that conversation via something like nocomment.