According to Wikipedia, AirPods Pro Gen 1 have 0.16 Wh of battery per AirPod (There's no data on Gen 2).
With 5 hours listening time, that gives a power draw of 0.032 watts or 32 milliwatts.
This answer https://electronics.stackexchange.com/a/640179 (I know, not the best source, but I'm just guesstimating anyways) gives a current of 1 mA at 5 V for an indicator LED. So the LED would need 5 mW. That increases power draw to 37 milliwatts and gives a new battery life of about 4 hours and 20 minutes.
If using 5 mA, which the answer calls "blindingly bright for some clear LEDs, even from 10 feet away", the LED would draw 25 mW and reduce listening time to only 2 hours and 50 minutes. The answer is also about non-diffuse LEDs so the indicator would only be visible from a narrow angle, but since it would point forward, that's probably fine. Making it diffuse would reduce perceived brightness again.
Brightness could be fine indoors, but outside with direct sun is probably harder. Since you would have the LED on both AirPods, you could probably expect that at least one of them is in the shadow at any time though.
But LEDs would probably take too much battery power.
Not sure if the new color eInk would be useful.