I downloaded the SiLabs CP21xx customizer, which fails to run on XP because it can't find SLAB_USB_SPI.dll sitting right next to it in the same folder.

Any other way to set unique serial numbers on these with Windows?

XP is refusing to work with more than one DP210x device at a time because the manufacturers of these ASIC modules either gives them all the same serial (block erupter) or doesn't set any of the programmable data at all (antminer).