For a macro pad, maybe. For a full-sized keyboard, that would be a lot of ADC channels. Looking at Digikey, ADC chips seem to be about $0.50 per input in bulk, and that would add a lot of BOM cost to the keyboard.

You might be able to do better if you instead feed the Hall output into a sample-and-hold filter constructed out of a much cheaper op-amp. I suspect that would be an interesting engineering challenge.