I made a site with bookmarklet quite some time ago that does something similar to that.. http://www.convotrack.com (was made more for twitter though)
Narrowing down to HN only and greasemonkey allowed me the following improvements:
* While the bookmarklet has to be actively opened by the user, HNCE pops up as a small tab whenever it is relevant.
* The comments in the bookmarklet are sorted chronologically (descending); Directly embedding the page with HNCE allows the hierarchy and ordering to be preserved.
* Another benefit of directly embedding the HN page is the ability to contribute to the conversation and up-rank the article.