I think it makes sense to have that bot as an actual separate site feature. Personally, it's not something I'd want to be mixed in with normal comments.
What about offering it as a service to the submitter from the submission page?
Or even a browser extension?
Or a separate browser just for that? Or maybe a Linux Distro? A HN Smartphone!
I have taken the precaution of searching for phrases in titles of older pages before submitting (I submit seldom) and have not submitted if previously discussed.