Just received some more tech info on the PP diff-lock system from Borg-Warner and hoping to run a more detailed tech feature in a future issue of Volkswagen Driver mag... ;-)
...as suspected though, it is not suitable for a straightforward retro-fit because the gearbox needs to have suitable mountings, unless you swap the 'box as well, of course.. so best you specify it at initial order, rather than regret it later...