I've added a link to post#6 in my page on Understanding Styles, if for no other reason than that I may be able to find it again. If I knew your real name, Italophile, I would credit you there, as well. You've made a valuable contribution to the Word community here. I expect it will also lead to other discoveries on things that can be done in XML.

There are a number of things that can be done in XML that can't be done in the UI. Most are well documented on the internet. One resource is John Korchok's series on XML Hacking at his Brandwares.com site. Whilst the focus of this is PowerPoint there are some Word specific articles, as well as articles on extending Office Theme colours that apply across the suite, and some PowerPoint specific techniques that can be applied to Word, e.g. shape locking.
I haven't included a direct link to the page as I have previously found that deep linking to his site can be unreliable.

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Thank you. I am well aware of John's contributions.

John Korchock - Best Practices - much about PowerPoint, some about Word - Start here and look at Older Posts
I have a number of bookmarks and like you have found that sometimes things move around. The link above is simply to the last post on the site and the way I titled it in my bookmarks.

Here is one on styles: Managing Styles with XML editing by John Korchok
It does not have the border trick you demonstrated.

I also have his book OOXML Hacking.

It doesn't need to. Once you understand that you can edit the xml to get around limitations you learn by experimentation. That's how John learned.

But you only experiment when you have a need to do something. Necessity is, after all, the mother of invention. I only learned to modify the border in this way because a client had the same intractable design issue. Similarly, I only experimented with the missing options in table styles because I needed to create a single table style that could be used to produce multiple table designs for different purposes by simply toggling the style options.