Hi @MattH, @Nobita.Kun has an addon which was created for me (and so far unreleased on here) which allows you to highlight posts. Currently the highlighting of the thread is done based on the number of likes the post has received (Supports post ratings addon too). Im sure if you ask him nicely and contribute towards his time in making that adjustment, he might build in a conditional that allows you to also select merged posts to be highlighted. How you may want to 'highlight' the post is controlled by style properties
Hopefully I did not misread your question, but I created an add-on which is as yet unreleased, which would allow you to collate posts by thread and date, instead of date alone, when using the Merge Threads tool.