software installed on it to support AirPlay. Not sure if it'll do mirroring, as I simply haven't tried. LMK if you need a confirmation.
The modding can be done with a USB stick, some software, an SSH client such as PuTTY, on the latest ATV1 firmware.
As far as not using a router goes, I have a jailbroken iPhone 4S, and am able to use MyWi to link the ATV and phone together. I don't have a tethering package with AT&T, so I can't speak to the built in hotspot feature. There is no reason that it shouldn't work though.
For me, a router might actually be more appropriate, as I don't actually want to tether. If you use a router and configure it to not assign a default gateway to clients, the iPhone will still use cellular data, but the wifi link you need will be there. Probably doesn't apply for you though, so just a general FYI.
Dude, I love what you've done. I'm a year behind your thinking. I want to run the atv3 in my motorhome using AirPlay etc.
I have stripped my Atv3 unit and pinned out the connections to the motherboard. It's a standard across the board 3.4vdc and the psu says 1.75amps.
I have managed to get it running off a 12v supply, which was more out of luck than judgement. You seem to have it sorted with the connections. Are you able to assist with the pins required to power up the unit. I've used 1 and 5.? Which seem to power it up but I haven't run it for any length of time as I struggled to connect my supply. Also how did you actually soldier up a connector block in side. I love to see some images if you willing to share.
I really look forward to hearing from you
I worked on this some months back, but as it stands, my latest plans are to solder the OEM power supply back together, as I simply don't know what type of circuit to build for it, and I got my in-car AirPlay support with an old, hacked up iPod touch instead :P
For power, I used this: http://amzn.com/B008BHAOQO Make sure you have a multimeter to set it up!
After mapping out the pins with simple continuity tests, I came across some really odd combinations in the way it pinned out, so I started testing it. Six of the pins are for primary power: Vcc and GND, three each (though I can't recall if that's pins 1-6 or 3-8). Indeed, connecting those pins to a 3.4v power source will start the unit, but it won't boot properly. If you attach an Ethernet cable, you can watch the interface reset over and over again. I also hooked up the power light
The other two pins are labelled, I think, E and F. My guess is that it uses what's called a "constant current" or "emitter-follower" circuit. I couldn't find information I could understand well enough to turn what I had into the proper source of power. I was about to ask on the /r/AskElectronics reddit page when I spontaneously lost interest in the project.
The pins are labeled pretty well on the power supply's board. Leave it unplugged for a few hours and its casing will come apart pretty easily if you need to get a look. This is the only picture I have that clearly shows the pins on the PSU:
Best of luck to ya.