Perhaps it was simply an available letter, as saghm said, but I've always imagined that at least part of the origin story is that it's a mnemonic for "invert" or "negative".
On an earlier page[1] the author mentions that they at one point "designed a more complex backplate which included […] a built in micro SD card reader, that you could read directly by plugging in a micro USB cable", among other things. Unfortunately, they go on to lament that this made the watch comically thick. As you say, the strap might be a good place to cram a USB interface without bulking up the watch body.