I will programm an Arduino Uno cluster via the RS485 bus. I use a shield with an RS485 to RX/TX converter, how i modify the bootloader to get the data and flash my Arduino.
There is no change needed for data transfer, since normally Tx/Rx are used. There is just USB/serial transceiver on the board which stands between PC and ATmega. The problem is how to ensure the reset over RS485 which starts the bootloader.
If you are using half duplex RS485, then you also need to control the direction of data flow - i.e. the tx/rx state of the transceiver.