In my opinion, in this particular case I think it was perfectly fine to add to this old thread. The thread isn't discussing a particular calibre feature/bug - instead it is asking for how people approach a particular issue. Your post didn't repeat anything that had been previously said and I think it is quite a clever suggestion. Unlike certain other topics there aren't loads of threads discussing the subject so I am sure future users would appreciate finding a range of suggestions in one thread.
Of course where things go wrong with posting on old threads is when they discuss calibre limitations, feature requests or bugs that are no longer relevant due to Calibre being updated so frequently. You did the due diligence by searching first, there wasn't a more recent thread you could find and the subject isn't one that is likely to involve a change in calibre any time soon that would obsolete the suggestions on it.
But dwanthny may disagree of course - it is less often I post outside the plugins subforum nowadays and he is one of the mods here. I just think that in this case creating a new post just to make your comment offers less value to future users browsing the forums than adding it to this thread which discusses the other alternatives
