Thanks so much, tuanphan, for your reply! I've really enjoyed reading some of your replies to other posts, btw 🙂 Fortunately, my client had a change of heart and so I didn't need to pursue this format further.
And thanks, PixelSoda, for the useful link. I'll bookmark it for future projects; I'd still love to know if Squarespace is planning to introduce a sidebar (including navigation menu!) option, as was offered in the Wells template for 7.0, in the future. That would probably make many folks here very happy!