Modern cameras return a serial number over usb, so that would be an obvious choice. The serial numbers themselves aren't very user friendly, but it it be very easy to map them to numbers or names.
I think that I should have put the "use case" more clearly.
Consider a rig of 40 cameras mounted on a long rail (or other frame).
What you'd like to do during commissioning is power up each camera (with CHDK installed) sequentially once they are mounted, test them, and work through them in position order assigning numbers from 1 to 40.
If I can do that at the camera (menu or @param ) during check out, it would be quick and easy.
Otherwise, I need the USB network working so that I can run back and forth between my multicam rig and computer and use some sort of software that notices which USB device just came online and that lets me associate its serial number with a number between 1-40 on the PC. Which then has to be send back to the camera so that it can display the number on the screen (see my screen shot above). Swapping out a camera or dissassemblng / reassembling the rig becomes more difficult too. And if you want to change PC's you need to be able to find and move the file that has the serial_number-to-camera_ID number.
If you've configured a networked system of small control devices you'll understand the challenge. But as I originally said, maybe there is an easier way to do this?
(I'd like to add the ability to optionaly include the serial number in DNG exif at some point anyway)
Semi-stealth digital watermark?
For the old cameras that don't return a serial, my plan for multicam was just to write a file with a unique id on the camera. I don't see any particular reason it should be in the CHDK menu.
Again, see my use case above. It's a question of ease of setup. A simple menu item change to CHDK or an additional PC software tool.
I've seen people use the owner name for book scanners, but you have to set that with canon tools or gphoto.
Is that feature / param value in every camera? Looking at the Canon menus, it seems to be only in my more recent and expensive cameras.