I've wanted to pull comments in a query on the PO side for a long time.  The problem is that comments aren't stored in any table.  The way I understand it is the comments are an XML attachment.  Maybe somebody more fluent in XML/Database lingo can explain better.  Suffice it to say, you can't use Addins to query comments.

I believe you can link them using SQL or perhaps MS Access query common field is L_INDEX - link the AP invoice record to the same field in the L_DAPI table - - I had to do this to find some corrupt comment records awhile back and it worked.

Here's an article on how comments/attachments work, with some SQL to show how to join--it's related to POs, but same concepts apply.
https://www.lawsonguru.com/Articles/View/tabid/346/ArticleId/549/Comment-Attachments-in-Lawson-v8.aspx