I tried these steps, and was successful, with some mods.
I am NOT an OSX developer, but I so happen to be using a Mac to play with my Tessel. I got tripped up on: brew install gyp
This had no end of problems, from me having an old developer preview of Xcode, to permissions problems on the /Library/Python directory.
After installing the latest Xcode, I was only left with the permissions problem on the Python library.
I brute forced my way through that by just making the directory writeable, which allowed me to finally install brew. It then had a symlink problem, but I brute forced that as well, and all seems to be fine.
I'd imagine a fresh out of the box mac with up to date Xcode would not run into these problems. Just so you know.
One Question: I think a serial number is written into the OTP area. That must be done in the firmware right? Is it the same for all teasels? If I'm modifying my firmware, what do I need to do in order to maintain a unique serial number in that area?