From what I read in the original passage. It seems like making the dock is not that easy compared to iPod. If I'm not mistaken, other that connecting both the LR output, you need to short circuit pin number 8 and 10, this enables the player to recognize that a cable is connected. And the next thing you need is to connect pin number 9 to a resistor of a certain value and to GND pin number 1 and 22.(from the passage it seems that 100k(variable), 47k(fixed), 33k(variable), 22k(fixed), 15k(variable), 10k(variable), 4.7k(fixed) should work, I'm not good with electronic circuit thing but I think it means variable resistor and fixed resistor) 47,33,10,4.7 might be a good choice cause the other seems to be compatible with recording function too. I think its best to choose the one dedicated to line out only.
havent tried it myself, I got terrible soldering skill T_T