Given that it has no voltage reference, would it not suffice to send A=0V and B=5V to send a logic 0 and A=5V B=0V to send a logic 1? The difference would sum up to -5V and 5V respectively, more than enough.
To determine direction, I would switch to output on the RS485 side when receiving a start bit on RS232 and back to input when receiving a stop bit. What say you?