We"d be happy about this :) Till now, Workers have to add a comment manually, so others know that a message has been sent again.
In addition to the activity log entry, is there the possibility also to add a notification comment within the conversation timeline which contains the link to the orignal message? I guess with the new toolbars (mail.read) we could easily implement something like this by ourself?

Once the activity log entry exists, you could do use the "record created/changed" event and have a bot comment/sticky on the message.

It might be a good idea to have an option on the "send again" feature to automatically leave a note on the message too. That would save the extra work.