HQ, I might not understand details of crescendo enough to understand some context I'm missing here.
I can answer a lot for bistro though, and what I've done to it. Classic Bistro doesn't use any timer or idle state for sleep modes. Well it does have waits that are done just by a tight do-nothing loop at full mcu power, so that's the cpu clock timer, but a not a "timer".
I've measured that at about 4mA current draw. The ADC adds about 0.15mA (150uA) so not much. I added idle sleeps, using the clock timer. that reduced it to about 2.1mA, which was important in the context of OTSM.
I also added power off sleep powered by capacitors, using the watchdog timer. This is for fully powered off clicks though, to time the clicks. You can't go to deep sleep while the light is on because PWM won't work.
The watchdog isn't that useful when the light is on because the fastest it will go is 16ms, and to program strobes you need faster than that anyway. So if you want a sleep mode for standard loops, you're going to need one that uses idle sleeps and the clock anyway, and at that point you've already got that, so why use the watchdog?
This idle sleep does increase moon mode battery life significantly (I'd guess about 50 to 70% for 1lumen, could be more if really designing just for that), but that's not why I added it. I added it because it takes 4ms to detect power off and that eats up a bunch of my cap power before I can get the mcu to full sleep, so I need full power to also be as low as possible.
Now the full sleeps that are watchdog timed are down around 8uA, but that requires BODS to get that low (and the right chips), otherwise you'll be up around 30uAs. But again, no PWM during those.
If you're intereste, it's here:
https://budgetlightforum.com/t/-/44344
The idle sleep timer is setup in fr-delay.h.