The Nexus 10 also has the fewest binary blobs of any recent Android device.
Also, if you want an Allwinner based tablet there are/will be many available. The main work would be to find one where you can get the schematics, and also spec sheets for all the peripherals. Some companies might be open to releasing that stuff if you pay them.
Spec wise, the problem is the A10 SoC itself, not the board - the boards for most of the A10 based devices are practically identical and very simple. It's particularly the Mali400 GPU that's posing a problem, and there the challenge in terms of specs might very well be ARM themselves.
That said, there's work ongoing to reverse engineer it (Linux works on it, but without hardware acceleration)
Also, if you want an Allwinner based tablet there are/will be many available. The main work would be to find one where you can get the schematics, and also spec sheets for all the peripherals. Some companies might be open to releasing that stuff if you pay them.