Eventually I got in contact with a guy who does 3D printing as a business, and managed to get a model knocked up. For now, the whole brake end will be a solid model with the windows merely painted in, but I hope to rework this at some point to have fully glazed windows like the coaches in the show. Trial print came through and certainly looks the part attached to the coach.
Still got to apply a coat of primer and eventually paint the model yet, but overall I'm quite pleased with how this little project is almost done with. The imminent release of Bachmann's Duck is also going to help, as I should have them completed by then giving him a smart rake of coaches.
For anyone interested, I do not intend to sell these parts on. I'm quite happy with having unique models that I can truly say are bespoke to my layout, and are as close as I can possibly get them to Ready to Run standards. It's going to be just my luck if Bachmann announce some for their 2013 range now!