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I do not know why, and cannot venture to guess why, Nikon did not respond. Except that they may be considering the idea, or considering how best to respond, perhaps.
Some might say, discouragingly, that a line can be imprinted onto a photograph with a trifling use of image editing software of even the simplest kind. But if such a line were visible during the taking of the photograph, it could be aligned properly in real time with the subject(s) and the details.
Such a line might also be inserted into the DSLR or SLR camera, also, in some fashions.
The try you made with a cotton thread, failed because it was not at a point of focus. That point of focus is usually within a lens, inside the optics. The only way to insert it properly so that it stands out in sharp focus, would be either to disassemble the lens and insert it in the right position, or to have the lens assembly pre-manufactured with the line/thread/wire contained within it.