The tin vans shared a body profile (loading gauge) with the later "laminate" coaches, but nothing much else. To do justice to models, you'd probably have to do almost all of their design as one-offs - certainly, if Murphy / IRM standards were to be attained. Makes them appear less viable, but I've said it before - virtually all 1960s trains, and late 50s diesel trains, are quite simply as unrealistic without one as without a locomotive.
The two BIG missing links in the 1950-75 period are AEC railcars and "tin vans". Many wagons too, but Provincial's cattle wagons, corrugated ("tin?") open wagons, and above all, the ubiquitous "H" vans, solved that through kits. Maybe when Leslie's sold out of them, someone will do RTR ones!
Your idea about a run of British Mk 1s along with CIE's "BR Vans" is an interesting one, indeed; I am sure we'd welcome IRM's thoughts on that type of idea.
One complication is that there was not a single type of "BR Van". The CIE genny vans were made from several different original varieties of British coach - thus, it would almost certainly only be feasible to do one type - but that would be fine.
Generator vans truly are the next stage needed, hopefully.