Seems like I won't need it after all
If anything I think names of people commenting should be hidden for 24 hours, so that you don't know if you're responding to the same person within a thread or not. That might help to cut down snark. (And it avoids the problems we have of toxic-anonymity).
I also don't think that HN should do this stuff, but should have a nice interface to allow people to create extensions and plugins and user scripts. (There are very many such already. Someone creating a nice curated list with screen shots and user reviews would get huge amounts of karma.)
Simplicity is nice but I suppose YMMV as to the line where utility becomes unnecessary complexity. Email notifications might be too much but even then sending them out as a daily digest might not be so spammy.
Though never checked if the API is open for having such an app or one could monitor this by user-id and trigger custom notifications(out of HN I mean) when there are nested replied.