Oh yeah, I'm aware that for any other chemistry you need an actual coulomb-counter to get a proper gas gauge. My suggestion was more about the specific matter of suppressing an unwanted and inaccurate low battery light.
Though maybe instead of trying to mod the circuit board, it would be easier to just disconnect the LED itself.
Alkaline at 100% charge is 1.5V, at 50% charge it's 1.2V, at 0% charge it's .9V.
NiMH at 100% charge is 1.35V, at 50% charge it's 1.35V, at 0% charge it's like 1.2V
Totally different discharge curves. There's no simple circuit to convert between the two.
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EDIT: For hard data, see:
Energizer NiMH : https://data.energizer.com/pdfs/nh15-2300gl1220.pdf
Energizer Alkaline: https://data.energizer.com/pdfs/e91gl0320.pdf