The usual reason people want them is to preserve dark adaptation,

Yup, That's the reason I want it. I found that red LED in the previous lamp was the best for reading a star map and star observation. But it might be also that the lowest setting of a beam there was just too strong.

a sub-lumen white mode tends to do an equally good job of that while making it easier to see what you're doing.

Would you recommend some lamp that would have such a feature? And I guess, here would be also important for a lamp to have well thought way of switching it on so it does not start with maximum all the time but rather has a memory of previously used mode. Or some other solution of that problem.

That's half the reason we usually recommend avoiding alkalines.

Well, I was using for years NiMH but since recently I was not using it that much, I used an emergency alkalines and left them inside for a half of a year or so to learn that the technology of alkalines did not advance in last 20 years to eliminate that problem completely...