I'm thinking I could run some sort of connected bus using Rasberry PI as extenders - USB devices -> Rasberry PI -> USB -> Rasberry PI -> USB devices. Or is it better to use Power over Ethernet ? I really don't want to have to run an IP stack and deal with all that complexity of security etc.
To get reliable USB over more than about 15 feet you'll have to use active extenders. I see a 31 foot active USB extension cable online for $15 right now. Good luck pulling the fat end through your walls, though. And it's only USB 2.0.
What are you going to do when all your old in-wall USB cables are obsolete and none of your new devices want to work with them?
These days you can transmit just about everything over Cat5e or Cat6 using converters, or convert it and transmit it over Ethernet.
Personally, I'd pull 2 or 3 Cat5e or Cat6 cables everywhere instead.
Here's just one example of allowing USB communications over an Ethernet network: http://www.startech.com/Networking-IO/USB-PS2/4-Port-USB-over-IP-Network-Hub-Adapter-USB-Ethernet-Device-Server~USB4000IP
Some confusion in the comments - this is USB in every room.
It sounds like you may want a Pi in every room
The System I show works like this:
[USB 3.0] [USB 3.0] [USB3.0]
| | /
Ethernet - Ethernet - Ethernet - [USB 3.0]
[USB 3.0] [USB 3.0]
USB 3.0 in 6 places
If you only want USB 2.0, the adapters are only $8
If you are really trying to build and evil (or good) Pi empire, just add a Pi where you want and you can have
USB Cam - Pi - USB - Ethernet - Ethernet - Computer
My suggestion was just adding the ethernet backbone:
The ethernet parts prices were to show how cheap it is to wire that way, not to avoid USB
The charging receptacles do not, can not and will not ever have data in them.
You can not have mains in the same box as low voltage data, any time you see it done, there is a separate inside the box.
You can buy these separators, so the mains and USB will share a faceplate, but inside, that is just not what is happening.
To get a roll of USB cable, without ends, you will need to order from china - try alibaba.com, it is not inexpensive.
But, however you wish, your circuit will work - the cost will shock you, you will have impressive reliability errors, adding connectors onto cable yourself will need soldering and not a crimp tool.
Just trying to help - best of luck
For $20 you can get a USB 3.0 to Gigabit ethernet adapter
Faster USB than the Pi, less expensive than the Pi, no need for an external power supply as the Pi would need.
Cat 6 cable is 7 cents a foot, the keystone less than $1.50, wall plate under $1, 10' 5e jumper less than $1.50
24 port Gigabit switch is less than $100.00
Run a cable from each room to a single point - fast, easy, cheap
Ethernet really is the way to go.
You do not want USB, really. The last USB standard was 3.1. The Pi B only supports 2.0, and has some comparably problems with 3.0, especially with hubs.
USB is just the wrong tool for the job. Don't force a screwdriver to do the work of a drill.
If ethernet security is an issue, run a new, wire only network. If you do not tie it into your current system, it will be about as safe as you can get.