I’m new to the hardware side but I did serial protocol work in software for years, and you can fake this with parity. The 9th bit is sent where the parity bit would be. Linux supports “stick” parity through the CMSPAR flag: with PARENB|CMSPAR|PARODD the parity bit is always 1 (mark), without PARODD it’s always 0 (space).

So: set mark parity, write the address byte, wait for it to leave (tcdrain), switch to space parity, write the data bytes. In Python:

import termios, serial
CMSPAR = 0o10000000000
s = serial.Serial('/dev/ttyAMA0', 9600)
def set_bit9(on):
a = termios.tcgetattr(s.fd)
a[2] |= termios.PARENB | CMSPAR
a[2] = a[2] | termios.PARODD if on else a[2] & ~termios.PARODD
termios.tcsetattr(s.fd, termios.TCSADRAIN, a)
set_bit9(True);  s.write(bytes([0x03])); s.flush()
set_bit9(False); s.write(bytes([0x10, 0x00]))

Receiving is the uglier half: turn on PARMRK and INPCK and a byte with the “wrong” parity arrives prefixed with 0xFF 0x00, which tells you its 9th bit. At 9600 the timing between the address and the data bytes gets a small gap from switching, most old devices don’t care. Use the PL011 UART (disable-bt), not the mini UART, the mini UART has no parity.

Then each indicator goes out as a child, something like {“scale-1”: {“values”: {“kg”: 12.45}}} on v1/gw/telemetry.