How to ensure proper power on reset in 1-Wire parasiticly supplied MCU?
As you've noticed, RC on the reset pin is not going to work when you have very slow voltage rise times.

Built-in brownout detection may work, but read the microprocessor's datasheet carefully to see if there is a minimum rise time on the supply voltage.

If you are not sure built-in brownout detection will work, you can use a "voltage supervisor" chip.  The basic supervisor is a three-terminal device that connects to power and ground, and pulls the output low when the power voltage is below the setpoint.  Connect it to reset and you're good to go.  You can buy these in a variety of voltages and turn-on delays.