Gating TTL UART Tx pin to simulate BREAK for arbitrary time periods
This circuit should work but I would make two serious recommendations.

1) Purge that 1N4148 diode and replace it with a Schottky type, such as a BAT54A, to get a much lower Vf.

2) What is the aversion to using a single logic gate part? Put in a single 2-input gate device to give you the ability to override the UART TTL output to generate the break condition.

If your project is a one-off type or hobby project you should be just fine with approach 1. On the other hand if you are making a commercial product I really recommend approach 2.