jatfrigo - Nice to hear from you. I might PM you soon, beware
The only masters I do are post production masters - for which I am the lead engineer.
Anyhow - I did work for a Sony Playstation 3 release, which is of course is a blu-ray device and a lot of the video games like to say that they are 7.1 sound so we did our work in 7.1
As you said - SONY has created their own lovely 7.1 with 4 speakers in the back, deemed "sides" and rears" and the normal LCR in the front with a sub of course.
As you probably found, the extra backs (or even when in front) are mostly useless! but whatever...
Can you be more specific of what the audio was? Was it music?
Anyhow - since Blu-Ray won the war against HD-DVD, I would suspect this might pop up a little more. I know that at very least, it's nice for companies to say their product is 7.1 - in my case, I chose to do very little unique sound placement in the back two.
I'd love to hear the details of who your master was for and what was in which back/side channels