It looks like the changes to the C code from the other answer do work (but off turns on the function keys, I had thought on would do that). I wanted to extend the code to auto-detect the keyboard for me and couldn't bear the thought of writing file searching code in C, so I ported the code to Perl 5:

#!/usr/bin/perl



use strict;

use warnings;



use constant HIDIOCGRAWINFO         => 2148026371;

use constant BUS_BLUETOOTH          =>          5;

use constant HID_VENDOR_ID_LOGITECH =>       1133;

use constant HID_DEVICE_ID_K760     =>     -19690;

use constant HID_DEVICE_ID_K760_ALT =>     -19688;

use constant HID_DEVICE_ID_K810     =>     -19687;



my %message = (

    HID_DEVICE_ID_K760() => {

        on   => (pack "C*", 0x10, 0xff, 0x05, 0x14, 0x00, 0x00, 0x00),

        off  => (pack "C*", 0x10, 0xff, 0x05, 0x14, 0x01, 0x00, 0x00),

    },

    HID_DEVICE_ID_K760_ALT() => {

        on   => (pack "C*", 0x10, 0xff, 0x05, 0x14, 0x00, 0x00, 0x00),

        off  => (pack "C*", 0x10, 0xff, 0x05, 0x14, 0x01, 0x00, 0x00),

    },

    HID_DEVICE_ID_K810() => {

        on   => (pack "C*", 0x10, 0xff, 0x06, 0x15, 0x00, 0x00, 0x00),

        off  => (pack "C*", 0x10, 0xff, 0x06, 0x15, 0x01, 0x00, 0x00),

    },

);



#die

#   "usage: $0 [on|off]

",

#   "	on  makes the media keys the default

",

#   "	off makes the function keys the default

"

#unless @ARGV == 1 and my ($choice) = $ARGV[0] =~ /^(on|off)$/;

my ($choice) = @ARGV ? $ARGV[0] =~ /^(on|off)$/ : "off";



my $device;



# find the first device we can set the option on

# TODO: add a parameter to directly specify a device

# TODO: add a parameter to make it set all devices

FILE_SEARCH:

for my $file (</sys/class/hidraw/hidraw*/device/uevent>) {

    open my $fh, "<", $file or do {

        warn "could not open $file: $!

";

        next;

    };



    while (<$fh>) {

        if (/HID_NAME=Logitech K(76|81)0/) {

            my ($hid_raw_name) = $file =~ m{(hidraw[^/]+)};

            $device = "/dev/$hid_raw_name";

            last FILE_SEARCH;

        }

    }

}



die "sorry, could not find a suported device on your machine

" unless $device;



# re-exec with sudo if we can't open the device

unless (-r $device and -w $device) {

    # unless we are already root

    exec "sudo", $^X, $0, @ARGV unless $> == 0;

}



open my $dev, "+<", $device or die "could not open device $device: $!

";



my $success = ioctl $dev, HIDIOCGRAWINFO, my $struct = "";



die "could not determine if $device is supported

" unless $success;



my ($bus_type, $vendor, $product) = unpack "Lss", $struct;



die "detected device $device is not a Bluetooth device

"

    unless $bus_type == BUS_BLUETOOTH;



die "detected device $device is not a Logitech product

"

    unless $vendor == HID_VENDOR_ID_LOGITECH;



die "detected device $device is not a supported product

"

    unless exists $message{$product};



syswrite $dev, $message{$product}{$choice};



close $dev;

Update: quick and dirty solution to the there being more than one device id for K760.