The old BD system (also honoured by LH and OS among others) was quite decent - they used to block seats next to *G passengers as long as the flight wasn't full, so middle seats were kept free as long as they weren't keeping a passenger from the flight. It worked over 50% of the time on the flights I used, and was extremely welcome.

However... The European system is much less orientated around booking-time seat assignment than the US, although this is gradually changing (not least as airlines realise they can squeeze a new revenue source out of this).