The Remote firmware has the Raspberry Pi locked out from using the serial connection, but with I2C1 it can be enabled. One of the status bits is for host lockout. I2C command 7 is used to set that status bit.

python3
import smbus
bus = smbus.SMBus(1)
#write_i2c_block_data(I2C_ADDR, I2C_COMMAND, DATA)
bus.write_i2c_block_data(42, 7, [0])
#read_i2c_block_data(I2C_ADDR, OFFSET, NUM_OF_BYTES)
#OFFSET is not implemented
print(bus.read_i2c_block_data(42,0, 2))
[7, 0]
exit()
picocom -b 38400 /dev/ttyAMA0
...
Terminal ready
/1/id?
# C-a, C-x.

Once the host lockout is clear the Raspberry Pi can bootload the address set with command 3 (see Remote). The Raspberry Pi will need to use its RTS handshack lines for avrdude to work.