While looking for something else, I found a reference to the /boot/overlays/README file, wherein it is written:

        act_led_trigger         Choose which activity the LED tracks.
                                Use "heartbeat" for a nice load indicator.
                                (default "mmc")

        act_led_activelow       Set to "on" to invert the sense of the LED
                                (default "off")

        act_led_gpio            Set which GPIO to use for the activity LED
                                (in case you want to connect it to an external
                                device)
                                (default "16" on a non-Plus board, "47" on a
                                Plus or Pi 2)

... snippage ...

        pwr_led_trigger
        pwr_led_activelow
        pwr_led_gpio
                                As for act_led_*, but using the PWR LED.
                                Not available on Model A/B boards.

Although the power LED isn’t (easily) visible through the Canakit cases I’m using (it’s under the barely visible hole in front of the small hole near the hacked RUN connector), turning it into a heartbeat pulse distinguishes the CPU’s “running” and “halted” states; whether it will also distinguish “crashed” is up for grabs.

It’s not at all clear what other choices you have.

To enable heartbeating, add this to /boot/config.txt:

# turn power LED into heartbeat
dtparam=pwr_led_trigger=heartbeat
#

I expected a simple 50% duty cycle heartbeat, but it’s an annoying double blink: long off / on / off / on / long off. Fortunately, it still isn’t (easily) visible …

While you have that file open, reduce the GPU memory to the absolute minimum for headless operation:

# minimal GPU memory for headless operation
gpu_mem=16
#

Some further ideas, including a way to turn off the HDMI interface.