[Jeza]

Early turbo OEM cars use to use this sort of system, eg Subaru Leone. Others on here have mentioned Saabs and metros?.

You may be able to modify a dissy from one of these to work with a VW, or pehaps just use the vaccum can to work the VW dissy advance retard.

The other option is to track down a late model VW dissy, as they had an advance retard type vaccum can on them for smog control.

Hugh MacInnes mentions a couple of these style solutions to retard the ignition in his Turbo book.

It would be worth seeing if you can work out how much retard you chosen dissy has before you fit it up though.

Cheers
Jeremy