I’m using susy, compass and sass.
I’m using Susy to specify the width
I’m not familiar with Susy but it seems to me that you are trying fix fundamental flaws in the HTML structure with a pseudo-element band-aid.
If a div needs to be the width of the browser…you just add it to the HTML outside of any wrapper and it’s done. If you need to limit the width of content inside that outer div you just add a centered div inside that outer div.
You can always start new wrappers whenever you want…that’s what my second demo attempted to show.
However,perhaps that’s something that Suzy won’t let you do.