Calgacus,
Now that I have access to a screen rather than on my not so Smart Phone I can see your original drawing in detail and understand your concept which is interesting but I'm not necessarily convinced that that was the reason for the central unsown section nor how the plaid was worn.
To put this piece in context, it is the
only complete surviving plaid that I know of that was definitely intended to be worn in a belted fashion rather than used as a domestic blanket of which there are a number of surviving examples. The plaid also post-dates the clan era by some 60+ years, roughly 2 generations, albeit that there might have been the odd old man still alive from the '45 era who might have worn a belted plaid. Portions of older belted plaids survive but there is no evidence of this technique in any of those pieces of any such construction or indeed evidence of drawstring ties. Nor is there portrait evidence or, and perhaps importantly, is there anything in any of the military writings, instructions etc to support this style pre-1800. The absence of evidence for something does not of course mean that it did not exist but nor does it mean it did.
Early writings and the drawings of people like Burt and Sandiman all suggest that the plaid was worn as a single layer in the way most re-enactors have assumed. Add to that the references to wool/plaids being treated with whale oil (nice!) to waterproof them makes me doubt your concept. I just question why?
The Highland Revival era was one that saw a good deal of cultural 'invention' not least in aspects of traditional dress. I'm still inclined to the view that this plaid is one of them, which does not preclude it having been worn as you suggest, I just doubt its use historically and feel that to wear it thus on-the-hill would affect some of the garment’s practical properties. I can see the attraction in a ‘fancy dress’ setting, which is what the Levee effectively was, as it would make the plaid less bulky and so would look smarter in polite company.
I won’t have access to the drawings I mention for a few weeks but in the meantime here’s a contemporary portrait of the chief wearing the plaid in a more fulsome manner that looks more like earlier portraits of a plaid being worn and which might go some way to support this idea that your interpretation could have been employed as a sort of Court Dress for special occasions where the effect was desired but the full benefits of the cloth not needed. I’m sure that this is something that we could toss around for ages but for me and based on surviving evidence this appears to be a one off..

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