Using pin 10 as the Chip/Slave Select line is a tradition, but there are no hardware requirements that demand it. All you need is a digital signal you can send LOW so the device in question knows it's supposed to listen to the data on the MOSI line. We've run into the same frustration you mentioned in your writeup enough that we try to build SS flexibility into our designs.
Your writeup looks good. I'll pass it along to the folks who manage the blog and see if they want to include it in our "cool things people are doing" feed.
The one piece of feedback I have for you is this: if it ain't ugly, it ain't a prototype.
Ugliness is a defining feature of anything new. When you make something new, you don't know exactly what you're building until it's done. The end result will be full of false starts, things you abandoned along the way, and things jammed in as you discovered the need for them. 'Pretty' only happens you know what the end result will be before you start.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.