RPI4 has an eeprom, however the other versions don't. I would be interested in particular in the RPI Zero.
Could you add an SPI EEPROM to the header/gpio pins in order to load u-boot or maybe even a kernel from it? A similar setup for other boards can be found here:
The header on the RPI has pins for SPI exposed, however, I don't know if they can be used for booting.