The number isn't in the broadcast signal, it's overlaid by the decoding box. It's going to be a number tied to the unique viewing card number (which is tied uniquely to a subscriber).
I wonder how easily the firmware can be downgraded (or downgraded) to remove that behaviour. Though as noted in the sibling thread, it's more likely easier to crop the video output.
I wouldn't like to bet against the number appearing right in the centre of the screen for a fraction of a second, if there were any particularly high-value programming being illegally streamed.
I'm only aware of one time that a legitimate box was encouraged to run a lightly-patched version of the software, and that involved a JTAG-type attack on a specific model of set top box. I believe that lesson has been learned in current models.
(2012)