Also known as: I'm a blithering idiot.
I spent way too long overthinking this problem and only finding solutions with floats (I have very specific cases where I'll use floats for layout, also I could have missed anything that didn't use floats for layout as I was stupidly staying up way too late when working on this). The way to do it is actually embarrassingly easy. I'll be using it for the header and footer (after I've installed Sassy, prepro and phpsass).
The above image is a screenshot of the header image on my local as seen in Opera (because it was the only one that showed the outlines regardless of the z-index). The middle columns are 950px wide (specifically set in the footer due to it being a background image, not set in the header as the image isn't backgrounded and the div sizes itself to it).
What I specifically needed was for the continuation images to start from the edges of the middle div (the left continuation image of the header and both footer continuations are uneven) and stretch to the edges of the container (which is the same width as the browser window, however wide that may be).