@doug - please, suggest away. I’ve used the MeshLabels mod on my Mk3 stock, but had originally planned to just manually duplicate some MDL files to provide a second RK and two more RTOs.

Well, we tend to think of .mdl files (and .mtl files for that matter) as being static configuration files. They are configuration files, and once the game loads, they are static. However, while the game is loading, we can make changes on the fly to these files.

If you have a look at any of the locomotive numbering mods that I've been putting up recently, you'll see that the model files are looking up their own name, using a lua debugging command.
The numeric portion of the name gets plugged into a string to produce the name of a mesh file, as I'm rolling my own mesh labels here. In each individual mod, the contents of the .mdl files are exactly the same. The only difference is the name of the file.

However, there would be nothing stopping you from doing the same thing to generate an .mtl file name, rather than a mesh file name. So, you get one self-configuring .mdl file set up the way you want, and then you just make copies of the file. You don't have to make any changes in the file, you just pick your names to match the material file you're going to use.

Also, if you're thinking of large numbers of variants here (or would like to) I wrote myself a lua script that makes copies of files. I've been using it for both model files and for .tga icon files. The latter of course you can do with the model editor, although it is rather slow by comparison.
You have to take the time to set it up for each duplication project, so it's only really an attractive option if you're making more than a dozen or so copies of the same file. If you're interested in using it, I can certainly share it with you.

Also, I seem to have got the hang of generating mesh labels, so if you're looking for a particular font, or a somewhat expanded character set, let me know. Given a list of characters (or any text items for that matter) and a font, I can generate the mesh files in a about half an hour... They're not explicitly mapped onto textures, so they really need to be used with solid colours, but I haven't found that to be a limitation so far.

I later had another thought...
Would a simple 2D square mesh item, mapped to a single texture, be of any value to you for this? Might make it easier than baking a whole new texture for each variant.

Thanks for so much useful info Doug - very much appreciated. I don't have the software or skills to do any actual modelling (3D or 2D), hence why my work is liveries and reskins.

For this set, I'm only envisaging a second RK kitchen car for each rake and two more RTOs, as the rest of the formation consists of unique items.

As the numbers are shared between the red/blue variants, I was initially looking at another model of the RK and two further duplicates of the RTO.

OK. Probably not much, if any, advantage to doing it programmatically then, if you're looking at half a dozen units all in.
I'm with you on the modelling skills. I've managed to create some basic shapes in Sketchup and import them into Blender, and whence to the Model Editor. My modelling toolset is therefore comprised of mesh labels and name/number plates!