Toggle the keyboard language by *holding* a button

It should stick even on Mountain Lion.

Two suggestions.

First, you might use Onyx to validate your preferences.

Second, try deleting ~/Library/Preferences/com.apple.systempreferences.plist and logging out and logging in again.

In either case, the pref file might be messed up and that could prevent it from being saved.

I've figured out how to do this. In short, you must send a "Feature Report" consisting of the bytes 0x9, 0x0, 0x0, 0x0 to the appropriate hidraw device as root.

You can find the right hidraw device with this command:

dmesg | grep Apple | grep Keyboard | grep input0 | tail -1 | sed -e 's/.*hidraw\([[:digit:]]\+\).*/\/dev\/hidraw\1/'

The code to send the magic control packet is below. Compiles with gcc, takes the hidraw device as parameter. So the entire flow is:

1. save the code below as disable-capslock-delay.c
2. gcc -o disable-capslock-delay disable-capslock-delay.c
3. HIDDEVICE=$(dmesg | grep Apple | grep Keyboard | grep input0 | tail -1 | sed -e 's/.*hidraw\([[:digit:]]\+\).*/\/dev\/hidraw\1/')
4. sudo ./disable-capslock-delay $HIDDEVICE

Steps 3 and 4 have to be done every time you reboot (or unplug and re-plug the keyboard); you can put them into /etc/rc.local (or your distro's equivalent) to execute them at boot (you don't need sudo in that case; and you might want to move the compiled binary into /usr/local/sbin/ or something).

I've put in some safety checks for vendor ID, device ID, and report descriptor length. You may have to change the latter two if your model differs from mine.

#include <linux/hidraw.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>


int main(int argc, char **argv) {
  if (argc != 2 || strcmp(argv[1], "-h") == 0) {
    printf("Pass a hidraw device as the first and only parameter!\n");
    printf("You may find the right device with:\n");
    printf("  dmesg | grep Apple | grep Keyboard | grep input0 | tail -1 | "
           "sed -e 's/.hidraw\([[:digit:]]\+\)./\/dev\/hidraw\1/'\n");
    return 1;
  }
  int fd, i, res, desc_size = 0;
  char buf[256];
  struct hidraw_devinfo info;
  char *device = argv[1];
  fd = open(device, O_RDWR | O_NONBLOCK);
  if (fd < 0) {
    perror("Unable to open device");
    return 1;
  }
  memset(&info, 0, sizeof(info));
  memset(buf, 0, sizeof(buf));
  // Get Report Descriptor Size
  res = ioctl(fd, HIDIOCGRDESCSIZE, &desc_size);
  if (res < 0) {
    perror("HIDIOCGRDESCSIZE");
  }
  if (desc_size != 75) {
    printf("Error: unexpected descriptor size %d; you've probably got "
           "the wrong hidraw device!\n", desc_size);
    return 1;
  }
  // Get Raw Info
  res = ioctl(fd, HIDIOCGRAWINFO, &info);
  if (res < 0) {
    perror("HIDIOCGRAWINFO");
  } else {
    if (info.vendor != 0x05ac) {
      printf("Error: Wrong vendor ID, make sure you got the right "
             "hidraw device!\n");
      return 1;
    }
    if (info.product != 0x0250) {
      printf("Warning: Unknown product ID 0x%x!\n", info.product);
    }
  }
  // Get Feature
  buf[0] = 0x09;  // Report Number
  res = ioctl(fd, HIDIOCGFEATURE(256), buf);
  if (res < 0) {
    perror("HIDIOCGFEATURE");
  } else {
    printf("HID Feature Report (before change):\n\t");
    for (i = 0; i < res; i++) printf("%hhx ", buf[i]);
    puts("\n");
  }
  // Set Feature
  buf[0] = 0x09;  // Report Number
  buf[1] = 0x00;  // Report data
  buf[2] = 0x00;  // padding
  buf[3] = 0x00;  // padding
  res = ioctl(fd, HIDIOCSFEATURE(4), buf);
  if (res < 0) {
    perror("HIDIOCSFEATURE");
  } else {
    printf("Caps lock delay disabled.\n");
  }
  // Get Feature
  buf[0] = 0x09;  // Report Number
  res = ioctl(fd, HIDIOCGFEATURE(256), buf);
  if (res < 0) {
    perror("HIDIOCGFEATURE");
  } else {
    printf("HID Feature Report (after change):\n\t");
    for (i = 0; i < res; i++) printf("%hhx ", buf[i]);
    puts("\n");
  }
  close(fd);
  return 0;
}