Are Passbook Passes Available Outside the Sandbox?
I wrote to Macroplant, who make the iExplorer utility that I bought some time ago, and they pointed me to the fact that Passbook Cards are available in the backup data.

When using iExplorer, they're located in the 'Backup Explorer' section, under HomeDomain/Library/Passes/Cards, the Backup Explorer section is apparently a logical mapping of ~/Library/Application Support/MobileSync/Backup/.

That's good enough for me, since data is routinely backed up. I can backup and instantly access my current cards.

Thank you Mike, on the Macroplant Support Team!