Hello, I've seen some posts regarding this but none lead to any development.
I think this theoretically would be possible, because few amp+dac combo devices like Sony PH-1, Fostex HP-P1 (for ex.) are able to bypass iPod's internal DAC and use their own hardware. I've read that this is possible by the apple authentication chip that was manufactured when these companies had a deal with apple. So it could be possible to reverse engineer the key from these authentication chips so Rockbox can use it in software to gain access to iPod's digital stream and output it via 30pin adapter.
It's probably way more times more complicated, than In theory, but I'd love to hear input from others.
If came to fruition then iPod would serve as a media and playlist device and would no longer be held back by it's dac, and would be able to utilize way more modern hardware, there are a lot of good and affordable external amplifier+dac devices on the market.