Here's my schematic for the HDMI adapter board, feel free to check it out to see if i have made any errors. The adapter is taking its input directly from the 20 pin header that Jens has specified he is using. The Audio header is a simple 2 pin header (taking advantage of 2 unused pins from the cable, plus one of the existing ground pins) that you can tie in if you are using a modded HDMI cable.
The actual board itself will be the same size as my previous vga board and will be able to be mounted using 2 holes. (taking advantages of holes already in the A1200 motherboard).