Afraid not, jbt. Altering a shape file requires the 3D file and even if there were ways to do that, you are then crossing the modeling line by altering another's work in ways beyond licensed skinning.
The good news is I believe I've found some period (prototype) models to paint. At least I have some to look at.
Please keep in mind that even though a model may look OK, whether or not it can be reskinned depends entirely on how the textures have been mapped to the 3D model i.e., if a part that needs to be lettered is mapped to be stretched to fit with a solid color, all bets are off. Texture mapping can also be difficult to dissect. In the end, having a good model is only the first part of what can be a frustrating skinning adventure which ultimately fails.