I now can share my own experience: with FP6 and /e/OS 3.0.4.
Indeed, there is no settings/feature for native always-on-display (or I could not find).
And indeed LED Blinker App (free version) works out of the box and can be used as LED replacement.
BUT: because of the missing AOD feature on this OS build LED Blinker has to use a different technique to show the blinking symbols, and this eats quite a huge amount of the battery.
I did a short test and let LED-Blinker blink for a whole day (“WiFi connected”) and it consumed about 40% battery for about 15h (so roughly 2-3% per hour.
When I disable this notification the standby drain of the FP6 is at about 5-10% per DAY!
One can adjust LED Blinker to stop after a certain time (e.g. 3h) and so the battery drain should be limited. BUT: it is still a drawback in my opinion, although a minor one than I expected.
I would be interested, if this observation is different on stock FP-OS with native AOD-support. Does it save battery for this scenario?
All in all I am not completely satisfied with the settings LED Blinker offer. E.g the blinking intervall can only be adjusted with one value, e.g. “3000ms” and it is slowly blinking within 3 seconds but the “display on” time is quite high (2.5s?). I would prefer just a short “blink/burst” each 3 seconds to gain attention but save more battery. But it seems not possible with this app.