I fitted the A1 wheel to my A2 today, sorry for the delay, you can breath now
The Audi wiring diagrams are pretty (very) poor (both ETKA and ErWin). I can't find a wiring diagram for the standard A2 wheel, the A2 MFSW diagram is OK but the A1 MFSW wiring diagrams have no colour codes, no pin numbers and no LIN identification at all. So its not a case of looking to Audi for the info.
The A1 and A2 airbags have two pins on them and for the life of me, I can't find anywhere on the internet that describes how the trigger circuit works so I can't work out which pin goes to which wire and what might happen if I got them the wrong way round!
The conversion is a three stage affair. Stage 1, get the airbag connected with no warning light and the horn working. Stage 2 get the buttons controlling something. Stage 3 get the buttons to illuminate and dim etc with the dash.
Stage 1 is relatively easy as long as you can use a soldering iron.
BUT DO THIS AT YOUR OWN PERIL AND READ THE ABOVE POST REGARDING THE RISKS... MAKE SURE YOU DISCONNECT YOUR BATTERY AND LEAVE THE CAR TO STAND FOR AT LEAST 30 MINUTES BEFORE DISCONNECTING OR CONNECTING AN AIRBAG, THEY ARE SMALL BOMBS AFTER ALL.
Remove the A1 MFSW loom from the airbag so you can work on it and then cut its yellow connector off which leaves 6 wires. Of these, you can tape up the 2 LIN Bus ones (the 3rd LIN Bus wire is shared with the horn). Then taking your old A2 yellow connector or a new one from Audi, cut the connector off its wires. Then connect the x4 A1 wires to the A2 slip ring connector. Two airbag wires, plus the horn 12v and earth. Once you have done that, re-attach the loom to the A1 airbag and fit it to the wheel et voila. I have done this, no warning light and horn works - see pic below.
Stage 2
The A2 slip ring connector has 7 pins as standard, of which I used 4 in Stage 1. The other 3 are for the buttons, the LIN Bus. This has a DATA line (bi-directional), an earth and a 12v power. On the A2 these 3 lines are separate to the horn. But on the A1 one of the LIN wires is shared with the horn. I think its the earth connection, but it could be the 12v power or the LIN Data sharing the horn 12v...I'm not sure yet...so the A1 only has 6 wires at its connector (there is a Y splitter in the loom so 6 wires becomes 7).
Anyway, to make use of the buttons, the 2 wires left dangling in Stage 1 need to also be connected through the slip ring and the 3rd wire needs to be run from either the horn 12v or the horn earth (see above) to the slip ring so all 7 pins are used. One pin goes to the 5 pin standard A2 connector (LIN Data) the other two need to go to another connector (there is a 2nd loom in the A2 MFSW) which sits above the yellow one, in the slip ring. Perhaps an owner of an A2 MFSW can post some picks of their slip ring with looms in place?
The LIN signal then enters the car where for stage 2, you could make a bit of hardware using an Arduino or Raspberry Pi which listens to the LIN Bus (LIN will probably be 12v so a circuit needs to sort that out to a suitable level) and then when a button on the wheel is pressed some software can broadcast a key press via Bluetooth (RPi's have Bluetooth built in now) to directly control a Smart Phone.
Stage 3
Stage 2 is about processing button presses coming FROM the steering wheel. To get the lights in the buttons to come on and dim etc we need to talk TO the steering wheel. This is more complex as you need to use a language that the button controller in the steering wheel understands - for this its probably best to use the Audi A1 MFSW controller which talks over the LIN Bus to the wheel controller to listen for button presses passing these on over the CAN Bus to the audio system (or phone etc). Plus, it talks to the steering wheel, to tell it what brightness to have its illumination. The controller will need coding for this.
Footnote:
Sorry I haven't mentioned what wires do what, but your wire colours may be different to mine and without a reference from Audi as to what exactly everything does its a bit trial and error. I took the risk and made an educated guess as to which way round the airbag wires needed to go. I THINK (try at your own peril) that if I got them the wrong way round it would either work anyway OR I would just get a warning light but there was a chance it would go BANG! It worked for me so I don't really have an answer.
My sport wheel had coloured wires in its wheel loom, the MFSW loom I bought from Audi had all black wires for LIN and horn (great) and two black and white striped wires (i.e. both the same) for the airbag (wonderful). The A1 had green and black for the airbag, brown and blue for the horn plus brown, white and purple for the LIN Bus (the two browns connecting into a grey) ... But the Audi A1 wiring diagram has no colour references on it (ridiculous)!!! So these colours COULD mean nothing on another wheel.
I doubt I will ever have time to do something meaningful with the buttons, but I will post here if I do. Happy New Year