@Victor Ronin had the right idea, in the Private Framework iPodUI
I found a class IUAccessoryEventHandler
containing methods like
- (void)handleDownArrowEvent;
- (void)handleUpArrowEvent
- (void)handleSelectEvent
When I hooked into them with a Theos tweak and inserted logging details, I was able to see that these methods were the ones being called when an external device was sending the commands.
Update: Unfortunately, the iPodUI
framework were only available in iOS 5 and 6, according to the iPhone Wiki. Not sure if or where these methods are still available.