When you lock a post, you also lock out users from being able to flag for moderator attention on said post.
It would be a useful feature to allow flagging from the post if something has changed or needs to be changed with the locking of the post. It would allow a user to explain their reasoning within the relevance of the post/page itself.
I found out due to a question that needed to be locked/closed due to some answer spam issues. It was a legit question and I did not want the OP to feel like he had done anything wrong.
I posted a comment explaining why I did this and wanted users to be able to flag the question if something legitimate needed to be added. That is when I noticed that you can't flag the question after being locked.
Nothing major, just an idea.