If were intending to use this I would not recommend making an "empty post"
Instead when someone has made a "thank you" a table could be put directly above the first topic on every page (basically between the top of the top post and the bar with the topic title in - either that or stick it at the bottom of each page - but I would think it needs to be on each page.
Or... maybe it would be better to put in the top of each page something simple like "X people have given thanks for this thread" which would be a link to a "thank you page" which would then list all the people who have given their thanks. This would be more appropriate in a very busy forum.
As far as difficulty in doing it goes I think I would just add another column to the topics table and store all the usernames of the members who have given thanks in that column comma deliminated - and read it in at the start of the page... hope this helps
