[upwardbound]

Do those charge pumps use external capacitors? If so, you could disconnect/float those capacitors, or if that would damage or glitch the chip, you could replace the real capacitors with some circuit like a voltage regulator + diode that would be designed to provide the charge pump output rail with a voltage that's high enough not to glitch the chip but low enough to be unsuccessful in writing to the flash. Would one of those ideas work, and allow the retrofit you envision to be designed with existing flash memory silicon + a few additional components?

[Scoundreller]

I believe they’re on-die capacitors for the charge pumps these days. The external ones are just for smoothening.

Probably a method to retrofit write/erase protection would be to just do power analysis and cap the current the flash chip can receive. Or shut it down if that works for you.

Not sure if they’re intelligent enough to run their charge pumps slower under compromised electrical conditions. Or if they’ll go haywire if they can’t do idle-time wear levelling/block erases.