I am attempting to work out, conceptually, how the use of a neutral density filter effects the resulting captured image.
Do ND filters allow a camera to capture higher contrast scenes (without clipping the highlights) than would otherwise be possible? Or do they just decrease the amount of light uniformly so that highlight detail is gained at the expense of shadow detail being lost?
Put more technically, is the effect of a ND filter more like a linear mapping or a clipping of the low light levels? See the figure below for examples of each. 
Answer
Simple answer: no, ND filters don't increase dynamic range.
In zone system, an ND filter just moves exposure of scene elements n stops lower. Everything that was in the lowest n zones captured without the filter, gets clipped off as black.
I can think of two scenarios where an ND filter might increase dynamic range, but I wouldn't use it for that:
- low quality ND filters will reduce contrast, so dynamic range increases a little (on expense of image quality)
- film is somewhat forgiving of underexposure, so you might gain more highlights and still preserve some shadow detail by underexposing; often you could achieve the same by simply adjusting exposure parameters
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