
This won't work if there is a checksum on the keys you don't know about.
Neither will registering a clipper chip and then substituting a
"counterfeit" one if the serial number contains a checksum.

Chances are the government has thought of this, and "just anyone" won't be
permitted access to enough of the internals to make a "fake" clipper chip.
Chances are that the government has classified some details of the internals
at a very high level, and manufacturers are required to observe security
safeguards and clearances corresponding to that level.

While not perfect, it would prevent Joe Hacker from rolling his own spoofing
chip, since not many counterfeiters can survive a background investigation.
Some of the more notorious self-styled dissidents here, for example, have
already got their heads so high above the tall grass that they'd have little
chance of getting a clearance.

Of course it's not impossible that someone do this, but probably extremely
difficult. Chances are there will also be very heavy criminal penalties for
phony clipper chips, on some national security/classified info grounds or
other, thus making the game not worth the candle.
