If you can use a momentary firmware, I already have something you could drop in. The Ferrero_Rocher/* firmwares already know how to control a couple of battery status LEDs which give realtime status while “on” and stay dimly lit while “off”. There are three different interfaces available, depending on whether you want something like an improved STAR UI, smooth ramping UI, or an Olight Baton-like UI.
With the setup I’m using, the parasitic drain with standby light only uses 0.36mA, so it should last about a year on a 3100mAh cell. Or with the status indicator light in “bright” mode it’s 1.23mA or about 100 days.
The firmware is at the link in my sig, and hardware build details are here:
It’s a nice setup; it’s too bad more lights don’t work this way.