Hi vaderag,
Ok, before I discuss changing the plugin, let's briefly clarify the requirement.
Firstly the thread you referenced is an "old" one, things move fast in Calibre world. Since that time meme developed the
Kindle Collections plugin, and that offers a very easy way to create collections per author or other criteria.
Now if I understand the "issue" correctly, it is that on a Kindle if you sort by Author, the results are displayed in reverse publication date order (most recent first). So your request and that thread above is based around fudging the publication date so that the books will appear in series order descending, rather than the last in the series at the top. Have I got all that part correct?
The approach I and many others have taken is to not use Sort by Author on the Kindle. Once you get more than a page or two of books it is pretty unmanageable anyways. So instead we use sort by Title, and have a collection per author. We name the collections with a special prefix like _ or ~ to ensure that alphabetically as far as a title sort is concerned they appear at the top of the list. So in my case I create collections using meme's Kindle Collections plugin based on the author sort field in Calibre, with a ~ prefix, and a very special custom collection called _Reading_ which will always appear at the very top that I add the current book I am reading into.
How does this help you with series you may ask? Well the trick is that you want the series name to appear in the title of the book, so that Wheel of Time 01 - xxx is displayed before Wheel of Time 02 - xxx etc. This was suggested on that thread above.
What I didn't see mentioned was the best way to approach that. That is to use a
metadata plugboard (and not hack your title name inside Calibre which is a crude suggestion some people do). So once your plugboard is setup, when you send books to your Kindle the series name/number is prefixed before the title of the book when displayed on the Kindle. Note this doesn't affect how your books look in Calibre, nor the filename of the books, just the internal metadata stored with the MOBI files that the Kindle reads to display on the screen.
So why do all this? It means that you don't ever need to muck around with publication dates, you can keep them at their genuine values and not "wreck" them by putting them in the opposite order. If a new book comes out in a series, you don't ever have to fiddle with it's date either.
Does that all make sense, or are there reasons why you don't want to use collections. It just sounds the "wrong" approach to me to try to muck around with the publication date. Instead it is just one click on the Kindle Collections plugin every time you send new books to your Kindle, and then a Kindle restart to have the new collections file displayed, job done...