Building a UART adapter
Get hold of a "30 Pin Dock Connector Extender Adapter For iPod classic Nano Touch Mini", something like that. Then you can create a UART breakout like this:
These are great, have all the pins hooked up and are pretty easy to mod, this way you can get UART on the device and USB at the same time!
The iPod classic uses 3.3v UART @ 115200 so make sure your UART<->USB adapter supports that voltage. I haven't yet tried RX on the iPod side but will update when I've tested it. Currently Linux uses DEBUG_LL for UART, aka about 5 lines of ASM to check if the port is ready and transmit a byte at a time.
Fun anecdote: This is from a paper funded by Microsoft which explores how the Nike+iPod could be used by stalkers, they reverse engineered the protocol and explained how an iPod running Linux (iPod linux used to be a thing yknow) could be used in combination with one of these to easily track when someone comes/goes from a place... Fun!
More pin pictures:
The back of the connector has the iPod Tx (used for sending data from the iPod to your host) and ground. Please forgive my terrible soldering job!
The front of the connector has the iPod Rx.
Take care to note the adapter orientation, this is a pretty visual process...