I'm no expert in these things, but I'd imagine there would be a soft flag for possible fraud, that triggers something like text messages to customers confirming their purchases.

If within X minutes you see Y cards and Z of them are known to be stolen, you set the soft fraud flag on all of them. Values of X, Y and Z would be set based on historical data.

You could still test your stolen cards, of course - but 5 at a time, not 50 at a time.

This would require significant collaboration between banks, seems unlikely.