Abiility to comment on specific source lines in files/regions not present in diff hunks

I often find when reviewing a change that it would be useful to say: the patch addresses instance 1 of problem X in file Y, but there’s another similar instance 2 in file Z elsewhere in the same repository. Or “look at this line in file Z for the recommended pattern”.

Today there appears to be no way to add such comments in say the file source view or by navigating the repository, which I think would be very useful.