There used to be a magic answer, IIRC, but:
a/ I don't remember it
b/ It triggered a factory reset, IIRC, which might not be that fun.
If b isn't an issue, that was something like RESETMYKINDLE or something in that vein.
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That said, if you can access the device over SSH, you're golden, just wipe the password db, that should do the trick (it's, err, somewhere in /var/local ?
).
Take that with a grain of salt, I've never played with the PIN locking stuff.