While a goods brake might be a fairly natural follow on to the van, I think any speculation that choice of a goods vehicle might be influenced by the 'D' is misplaced .............. while it's unlikely this printing technology is capable of producing a full blown passenger vehicle yet ( what would you choose as a one off, anyway ? ) NPCS should be possible - and I'd vote for an SECR Special Cattle Van as they, too, lasted into BR days.