Well, I re-flashed the t520 using exactly the same USB image (didn't even re-create the USB key), and this time it behaved differently. Before, there were at least 3 reboots with the message about important changes being made, but this time it was a straight re-flash with one reboot.
I don't know whether it was the fact before I was moving from a x86 to x64 image, and this time there was already an x64 image in the flash. The only other thing I did different was to make sure the LAN cable was disconnected during the re-flash.
Whatever fixed it, I was prompted to set an Admin password on the second flash.
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