It's possible to add it, yes, BUT (as with the 16550, though this was fixed in the 650 and above) the control lines have no relationship to the UART, in that when the UART input fifo is full it will not drop RTS and if the transmit fifo is not empty it will not stop sending when CTS drops.

What does your app require? (bearing in mind that the in-car serial port doesn't have rts or cts even connected)

Hugo