Gordon, didn't you manage to sell that Voightlander !!!
There are inserts for all 9x12 plate holders to take film, they do come up for sale on ebay and Ole posted a link to a site selling them (I never had a reply to my email).
There were adaptors made in the US to adapt a 9x12 to take modern 5x4 double darkslides, this has to include a new focus screen as the film plane is shifted back from the original. There is reprted to be a semi-retired camera repairman in the US who still makes these adaptors.
Søren, you're in the right part of the world to find plate holders, search ebay.de remember your Logon details are the same for any Ebay site.
The plate holder film conversion insert is just a simple piece of black painted metal with edge folds to hold the film and I plan to make my own on my next trip back to the UK.