I think you already know this trick: connect DE/RE signals of RS485 half-duplex transceiver to TX *negated* signal from UART and connect TX input of transceiver to GND.
If the bus is not driven by anyone, A signal goes high and B signal goes low thanks to the pull-up/down resistors.
The trick uses the fact that the undriven bus and transmitting 1 is the same thing: A is high and B is low. So, when transmitting 1 the driver is disabled (DE=0), when transmitting 0 the driver is enabled (DE=1). Because TX is set to 0, A goes low and B goes high.
Of course this works in ideal and good conditions. When should I worry of this trick? I suspect the edges of 0 to 1 transitions are slowed down.
I don't use mega-baudrate, in this application I'm using 38400bps.