I don't know. If the free version doesn't work with it, then the pro version won't either.
ReReplacer can replace in any content rendered through Joomla and passed through the plugins.
Generally extensions that load content via ajax don't do that.
I am not sure how Jcomments works, but I guess that it loads the comments via javascript (ajax) and just pushes them to the page. So neither ReReplacer nor any other PHP based plugin can change that content.