It was apparently hidden in the lead-in area, but I can't find any information on how it was encoded. Some sources say "a hidden sector in the lead in" but that doesn't seem right, as there is nothing physically stopping a DVD burner with custom firmware from writing a hidden sector.

The disk key is small (40 bits) and I'm suspicious it's actually encoded as wobble frequency [0], like the PS1's copy protection scheme.

Because CD/DVD burners can't write wobble. Blank CDs/DVDs ship with a pre-made wobble in the pre-groove, which the burners use to determine the absolute position of the write laser.

[0] *https://en.wikipedia.org/wiki/Wobble_frequency

That was a late edition. I have working DVD drives that will happily read anything on a disc, even if they can’t decode it.

Newer drives I bought will refuse reading what they won’t decide themselves (e.g. wrong region).