The Retro Adapter is now available!
Main site: http://retro.world3.net
Tech site: http://denki.world3.net/retro_v2.html
Many, many thanks to Seeedstudio for help developing this project. The PCB service is fantastic! Please feel free to start selling PCBs and kits now. You might also want to do pre-programmed AVRs - I suggest using the bootloader. If you don’t have a suitable socket for the QTFP chip you can just hold it in place on the Retro Adapter PCB with your finger as I do 
All the necessary lines for the ISP are on the DB15 so you can easily make a programming adapter.
I am now selling kits and some ready-made adapters. This is the first project I have done, and I’m not sure how long I will offer the ready-made adapters for, because they take a while to assemble and I’m doing it in my spare time. I am also working on a Gamecube version which will use the same PCB.