But the bootloader does not send the device ID, does it?
I have no idea if it does or not. AVRDUDE gets rather annoyed if the ID is not available or is not the expected value so I assume the bootloader has to send the processor's ID to satiate AVRDUDE.
Does the bootloader mimic the built-in serial interface?
By "built-in serial interface" I assume you mean the interface provided by the processor. In which case, I have no idea. I don't know anything about the on-processor interface.