Nothing about a mask ROM makes the data unrecoverable. It's still Memory that can be Read. I'd be very surprised if there's not an exploit that will get the CP

Nothing about a mask ROM makes the data unrecoverable. It's still Memory that can be Read. I'd be very surprised if there's not an exploit that will get the CPU to barf up the full ROM contents. That's if there isn't a more direct way to read it. Even in the extremely unlikely case that it can't be read programmatically, you can always physically decode it with a microscope and a working eyeball. From there, it's "just" a matter of decompiling the machine code into something readable. It's not trivial, but it can be done by a single person in a reasonable timeframe.