The textures aren't completely full coverage, like most games. They already use a lot of projections and instancing; see
here for examples of Standard texturing. Most road cars can be treated as base models already; Standard race cars would be the only exclusions.
That projection method is basically how a livery editor UI would work, and is in fact how Forza's works (superficially); the final result is / was flattened into the base textures at lower resolution, however. That seems unnecessary on PS4 / XBone, depending on the complexity allowed.
What PD would have to control is what parts of the car can be painted on in a livery editor. That can be controlled with material IDs; the Standards have had theirs processed twice for the change in paint shaders in the last two games. They'd have been daft to miss the opportunity to identify (inverse) masking at that time, since a livery editor had been on the cards for some time.
Cars with existing liveries, i.e. race cars, are more of a challenge. I expect the base models are the answer to that challenge (they needn't be separate models if a livery editor is available). Making the base models is easy with new cars, you can structure the work so you pass through that point naturally, but retro fitting is extra work. Semi Premiums seem like good value, as a result.
EDIT: you could just replace the Standard race car base texture with something entirely new, if you've processed the geometry to provide the shading (curvature, panel gaps etc.) in hardware, instead of texels. Panel gaps are a minimum. Interesting.