And this is why using the TX/RX lines for a simple on/off signal is far more complex than necessary. Faking a byte requires timing with some precision[1], and that timing is dependant on the current configuration of the receiving system (the baud rate set for the serial port). Using a control line instead of trying to fake a byte is far easier.
Tim
[1] Though not as much precision as proper uart comms, as receiving (say) 00000111 instead of 00011111 is probably OK.