After the last project log, I posted my soundcard board to /r/printedcircuitboard, which was very helpful. Probably the single most helpful suggestion was to AC-couple the headphone jack instead of DC-coupling it. (The TI codec I chose can DC-couple the headphone port -- it provides pins that can be driven at some common voltage above ground, which the left and right channels are referenced to.) This should prevent short circuits if the headphone jack gets plugged into something that shares a ground with the 30-pin dock. It also freed up two outputs on the codec (the HPLCOM and HPRCOM pins can be used as outputs instead of as common pins). These extra outputs meant I could get rid of the analog switches I was using to select whether the codec's line out pins would be routed to the speaker amplifiers or to some headers (eventually meant to route to the 30-pin connector.)
I also started another Github project meant for breakout boards for the various connectors on the iPad's logic board. In particular, I've created breakouts for the AA03-S024VA1 connector (for the iPad's ambient light sensor) and the AXK770147G (which connects to the PCB that has the 30-pin connector and the wifi/BT radio).
After first doing a simple breakout for the AXK770147G, I made a second version where I went a little overboard, with impedance-matched differential pair traces (probably silly, since I'm breaking out to 0.1" headers), and deduplicated power pins. (The AXK770147G has a tiny 0.4mm pitch, so to carry 2A of USB power, it uses 7 pins. A single header pin will work just fine.) I also tried to pay attention to the signal return path for all interesting signals.
I've ordered these boards + a stencil for the soundcard. Hopefully everything works!
I think I've decided to keep the 30-pin connector. I had been mulling the idea of replacing it with a pair of USB-C ports, which would feel much more modern, but I kinda like the idea of rescuing old 30-pin accessories from eBay. I'll aim to specifically support the Digital AV Adapter (HDMI out, though it might take DisplayPort inputs from the dock connector?), the Camera Connection Kit (USB-A port and SD card readers), and the iPad Keyboard Dock (which also has line out). The Universal Dock is also cool - it has line out and supports the infrared remote control that comes with the Apple TV.