The following files are debug builds that I have made for various versions of iPod. Some of the builds could be unstable and might cause the iPod to crash. Use at your own risk. All these builds have been tested on my iPods but could still cause issues on yours.
If your accessory does not function with the daily build from Rockbox.org you could try to replace the installed copy of rockbox.ipod with the the relevant copy from here marked with debug in the name. This will allow you to dump the received and transmitted IAP commands to a disk file called logf.txt.20140406xxxx by going to main menu -> system -> debug (Keep Out) -> dump log file. Uploading this file might allow me or others to understand what needs to be changed in the IAP commands to allow your accessory to function with your iPod. You will need to update FS#9951 Accessory Bug Report with your results.
The current files were built from git on 2014-04-06 with a minor change to the logf function to append not overwrite and to create a file based on the compile date rather than being a fixed name.
iPod 1G Nano debug version of rockbox.ipod
ipod 1G Mini debug version of rockbox.ipod
iPod 2G Mini debug version of rockbox.ipod
iPod 4G Grey Scale debug vesion of rockbox.ipod
iPod 4G Color/Photo debug version of rockbox.ipod
iPod 5/5.5G Video debug version of rockbox.ipod
At some stage I might build newer versions of these but they do suffice for me to see shere the IAP commands are failing.