Any good way to automatically enable an RS-485 transmitter in hardware?
If you're using an ordinary asynchronous (UART) protocol, with a start bit at the beginning of every byte, one simple method is to use a monostable multivibrator (even the dreaded 555) set to the duration of a byte plus the start bit and about half the stop bit, and use that to enable the transmit driver.

If the multivibrator is retriggerable, you need to allow up to one full byte time from the end of a transmission before anyone else can transmit, because the multivibrator may have been retriggered by the last data bit if it was a zero.

If not, then it should shut off in the middle of each stop bit, and then trigger again on the next start bit.