After looking into how conceal works, it doesn't look like you can use it to "annotate" a line or dynamically change the text on the screen without actually changing the contents of the buffer. It can only be used to hide some pattern of text that's already in the buffer.
I would look into using popups, a separate pane, or something like vim-notebook.
I would look into using popups, a separate pane, or something like vim-notebook.