The JS in the page should attach an event to the link to append the new page at the bottom. Better: it should do it automatically when the scroll position is close enough to the end.

Bonus: the script should add a search box to search even in the content not yet in the page.

I googled if it's possible to intercept CTRL-F and handle it programmatically. It seems it is [1] but I don't know if I would do it: the user could be surprised to find a search box in a non standard position with a non standard behaviour. It reminds me of scrollbar hijacking.

[1] http://stackoverflow.com/questions/9159322/overriding-the-br...

What I would do is add a dialog the first time the user press CTRL+F. Asks if the user wants the default or the override. Save this in a cookie or in the database.

A simple (non-JS) "Search these comments" form would suffice, but I've never seen it implemented.