I asked on the mailing list docbook-apps, what I should do, and a helpful mate suggested to make use of . That was a pretty good suggestion, I was rather thankful for it, and I did that for a couple of presentations.
Introducing is a rather tedious process though (and remember: engineers are notoriously lazy, but as engineers they are also rather ingenious by nature), as you keep trying to replace the original long title by a fancy shorter one.
Recently and just of the blue it came to my mind, that there ought to be a stylesheet parameter determining the width of the navigational ToC. Its name is toc.width, I set it to 500, and there is no more need to abbreviate section titles.