There is no benefit in a unique challenge id number that is applicable across all polling locations.
Alternate way to think about it: If you want this (say on a form), then go ahead and prefix whatever numbers you want on the form before the Challenge ID component. The VCE will “automatically” incorporate these numbers – in fact it will be so seamless that you won’t even know that it is, uh, ignoring the prefix.
Suggestion: In order to support unique Challenge Id numbers that would be applicable across all polling locations as well as applicable devices, why not expand the Challenge Id number to include the precinct number, machine Id, as well as device code (ie. 0 - Ballot Station, 1 - Voter Card Encoder, 2 - Precinct Control Center, etc.)
On all devices, the precinct, machine Id, and device codes could be automatically incorporated, without having to manually enter these numbers. In all cases, the actually Challenge Id is taken from the 'Challenged Codes' list that Ken has assembled.
If Challenge Id numbers need not be unique, then the Challenge Id component need not be taken from the 'Challenged Codes' list
Question: Would the Voter Card Encoder be able to support the longer Challenge Id numbers?