"Good idea, easy to implement. Add a cheap red LED (based on GaAs, voltage drop about 1.6 V) and a resistor to a GPIO pin."

No need even for that on a Pi. In my RPi::WiringPi, I use a couple of command line commands to manipulate the power and disk I/O LEDs so that a user can identify by eye which Pi is the one currently being worked on (this was a user request I implemented). Since this is a Perl forum, I've left the CLI commands within the actual code itself. Essentially, the identify routine turns the power LED off completely, and turns the disk IO LED on solid. Using these LEDs to set them to non-standard configuration could easily indicate a problem with custom software.

sub io_led {
    my ($self, $tweak) = @_;
 
    if ($tweak){
        # stop disk activity from operating the green LED
        `echo none | sudo tee /sys/class/leds/led0/trigger`;
        # turn on the green LED full-time
        `echo 1 | sudo tee /sys/class/leds/led0/brightness`;
    }
    else {
        # turn off the green LED from being on full-time
        `echo 0 | sudo tee /sys/class/leds/led0/brightness`;
        # start disk activity operating the green LED
        `echo mmc0 | sudo tee /sys/class/leds/led0/trigger`;
    }
}
sub pwr_led {
    my ($self, $tweak) = @_;
 
    if ($tweak){
        # turn off the red power LED
        `echo 0 | sudo tee /sys/class/leds/led1/brightness`;
    }
    else {
        # restore default power LED operation
        `echo input | sudo tee /sys/class/leds/led1/trigger`;
    }
}
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