Here's a guy from the Netherlands who did just that with a Golf IV from 2001 (!), well almost as he reached 31,7 km per litre. His goal was 30.
Mijn Bluemotion: zuinig rijden met Volkswagen Golf IV TDI | Zuinigste Golf IV in aanbouw
The website is only in Dutch and is no longer updated since he achieved his goal. He did it to make a point and to prove it could be done.
Changes he made are typical what you find also here on EM. Reduce weight, rolling resistance, taller endgear etc.
IMO VW charges a lot of extra money for their Bluemotion eco-range while in fact the technical changes are costing them small bucks.