Glen Akins alerted me to this little chip: The Microchip 25AA02E48T-I/OT. It's a pre-programmed 256x8 SPI flash chip. The chip is available as a SOT23-6 and is a standard SPI peripheral. The top 6 bytes (0xFA-0xFF) come pre-programmed with a globally unique EUI48 address. 38¢ Q:1 pricing at DigiKey. Mischief Managed.

To use it, you do an 8 byte SPI exchange: 0x03 0xFA followed by 6 "don't care" bytes. You get the MAC address by reading the values during the "don't care" bytes.