I'm working on an app that uses the UART for RS422 data transmission
You do not need to do that for RS422, only RS485
It seems to me that the the normal simple 8051/8052 parts have an unbuffered transmit which tells me that the TI flag is set after the byte has been transmitted (the Tx data register is empty). From that I am concluding that I can disable the Tx level shifting outputs on the next interrupt following my last message char (transmit complete). Does this seem to hold for most of the common (non-turbo or otherwise enhanced) parts?
If you read the 'bible' you would see that the TX interrupt happens in the middle of the stop bit = 1/2 bit time before transmission is done.

Erik