If we take the photo Skoot posted above as an example, and assuming I'm understanding things right), to add that number on the front of the trailer we would need to go into Blender and add it as a separate mesh which will then be called ingame via the locator. That's just for a number.
Now, I've seen this done on a few trucks in the past, and I agree it works as intended, but it is cumbersome to do. Even if it's just copying&pasting pre-existing stuff you made as a template, the average user might struggle with that system. You have to take into consideration that not everybody might have the same level of knowledge we do.
With that said, I'm thinking of potential solutions without having to butcher the model and perhaps you could see if it was possible to make a copy of the trailer, rescale it to be bigger than the real one by something like 1mm on all 3 X/Y/Z axis, lay it on top of the actual trailer and make it transparent. Then, it could be uv mapped as normal, but it would be effectively invisibile, other than the parts of it that would be painted. That way, maybe you could achieve the same result, but with the benefits of a more traditional uv mapping and skinning method.
Plus, I mean, these trailers might not be the same model as yours, but they do come in fully (or partially) wrapped liveries sometimes.
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