To answer my own question, I see that the Silicon Labs CP2102 device has an internal EEPROM which you can actually program yourself with your own serial number, and other info, using a utility from the silabs.com website.
http://www.silabs.com/products/interfac ... ridge.aspx
also, XRP (Hong Kong/Shenzen) is offering a USB-UART module with this chip for $7 shipped.
http://www.amazon.com/gp/product/B008BZ ... 00_s00_i00
the shipping is very slow, but next month I should know if this will work to give me unique serial numbers- seems like it should.