Agora eu descobri como conectar / tocar em eventos de teclado no OS X em um nível baixo:Como tocar (ligar) de F7 a F12 e Ligar / Ejetar em um teclado do MacBook

Imprimindo o código dessa resposta:

// compile and run from the commandline with:
//    clang  -framework coreFoundation  -framework IOKit  ./HID.c  -o hid
//    sudo ./hid

// This code works with the IOHID library to get notified of keys.
//   Still haven't figured out how to truly intercept with
//   substitution.

#include <IOKit/hid/IOHIDValue.h>
#include <IOKit/hid/IOHIDManager.h>

void myHIDKeyboardCallback( void* context,  IOReturn result,  void* sender,  IOHIDValueRef value )
{
    IOHIDElementRef elem = IOHIDValueGetElement( value );

    if (IOHIDElementGetUsagePage(elem) != 0x07)
        return;

    uint32_t scancode = IOHIDElementGetUsage( elem );

    if (scancode < 4 || scancode > 231)
        return;

    long pressed = IOHIDValueGetIntegerValue( value );

    printf( "scancode: %d, pressed: %ld\n", scancode, pressed );
}


CFMutableDictionaryRef myCreateDeviceMatchingDictionary( UInt32 usagePage,  UInt32 usage )
{
    CFMutableDictionaryRef dict = CFDictionaryCreateMutable(
                                                            kCFAllocatorDefault, 0
                                                        , & kCFTypeDictionaryKeyCallBacks
                                                        , & kCFTypeDictionaryValueCallBacks );
    if ( ! dict )
        return NULL;

    CFNumberRef pageNumberRef = CFNumberCreate( kCFAllocatorDefault, kCFNumberIntType, & usagePage );
    if ( ! pageNumberRef ) {
        CFRelease( dict );
        return NULL;
    }

    CFDictionarySetValue( dict, CFSTR(kIOHIDDeviceUsagePageKey), pageNumberRef );
    CFRelease( pageNumberRef );

    CFNumberRef usageNumberRef = CFNumberCreate( kCFAllocatorDefault, kCFNumberIntType, & usage );

    if ( ! usageNumberRef ) {
        CFRelease( dict );
        return NULL;
    }

    CFDictionarySetValue( dict, CFSTR(kIOHIDDeviceUsageKey), usageNumberRef );
    CFRelease( usageNumberRef );

    return dict;
}


int main(void)
{
    IOHIDManagerRef hidManager = IOHIDManagerCreate( kCFAllocatorDefault, kIOHIDOptionsTypeNone );

    CFArrayRef matches;
    {
        CFMutableDictionaryRef keyboard = myCreateDeviceMatchingDictionary( 0x01, 6 );
        CFMutableDictionaryRef keypad   = myCreateDeviceMatchingDictionary( 0x01, 7 );

        CFMutableDictionaryRef matchesList[] = { keyboard, keypad };

        matches = CFArrayCreate( kCFAllocatorDefault, (const void **)matchesList, 2, NULL );
    }

    IOHIDManagerSetDeviceMatchingMultiple( hidManager, matches );

    IOHIDManagerRegisterInputValueCallback( hidManager, myHIDKeyboardCallback, NULL );

    IOHIDManagerScheduleWithRunLoop( hidManager, CFRunLoopGetMain(), kCFRunLoopDefaultMode );

    IOHIDManagerOpen( hidManager, kIOHIDOptionsTypeNone );

    CFRunLoopRun(); // spins
}

Como posso (talvez adaptar esse código) identificar qual teclado é responsável por um evento específico?

O caso de uso é que estou planejando usar um teclado externo que será remapeado, mas, ao mesmo tempo, manter o mapeamento original do meu teclado MacBook embutido.

EDITAR:
Filtro OSX HID para teclado secundário?
https://github.com/candera/khordr/blob/master/src/c/keygrab/hid-scratch.c
http://ianjoker.googlecode.com/svn/trunk/Joker/Joker/hid_test.cpp
http://www.cplusplusdevelop.com/72_17345226/
http://www.cocoabuilder.com/archive/cocoa/229902-which-keyboard-barcode-scanner-did-the-event-come-from.html