It certainly gave me quite a headache in the process!
I'm afraid I never did discover how it's done in spite of looking at a number of different files; though I admit I'm not an expert at hunting down elusive data, especially when I don't know exactly what I'm looking for.
What I did find is that it isn't the value that was changed from 0 to 8, unless I was doing something wrong: that just comprised part of the file number it loads for a specific model and if an invalid number is given (i.e. file doesn't exist) it won't swap out the body part which gives the illusion that it does what we're trying to accomplish.
My conclusion was that it's (probably) not specified in any of the files I was looking at, be they ITL, AMR, ATR etc and nor is it done by the MOD (the model/mesh) file itself: no amount of trying to fiddle about with any of them produced the results I was looking for. So it's either specified elsewhere that we don't know about or it's some complex calculation based on more than one of the previous things.
To the best of my knowledge, nobody else has accomplished this either; or at least I wasn't able to find even one example of it. There are some mods doing the rounds that seem to have accomplished this but what they actually do is a complete retex of another item that already has the required properties. This may or may not be a useful solution, depending; for me I think the purist in me wanted to know how to accomplish it, as well as not wanting to "sacrifice" other piece of armour or clothing.