I just wanted to report that over the last few weeks, I have managed to find not only the 1988 book Modelling Maritime History by Malcolm Darch, but also the nine 1987 issues of Model Boats in which his Moshulu build is described in further detail and finally, to my great joy, full size, high resolution scans of Malcolm Darch's two 1:96 plans for said ship! 
The two Eric Newby classics, The Last Grain Race and Learning The Ropes, I already had since quite a number of years.

I have also been in contact with Mr Darch by e-mail, which he most kindly and helpfully took time to answer. Unfortunately, he no longer sells any of his plans, as he explained simply because getting them reproduced these days requires too substantial a trip.
However, some further googling turned up a set of his Moshulu plans at the Brunel Institue, attached to the SS Great Britain museum in Bristol, and an enquiry there gave the positive answer that their entire plans collection (some 7,000+ plans from the David MacGreagor collection, see **LINK**) has been digitised and that copies can be ordered at a reasonable rate.
As already mentioned, there'll probably be at least ten týears before I even get around to attemting a build, but at least I now have the essential documentation ready at hand … 
Mattias