An alternate way is to set RTS via digitalWrite (when buffer is full).. supersimple, reliable, no fiddling with interrupts and bugs in hardware....
...can use any pin and every Serial on T3.. works with fifo and without.
edit: used this with my "audiostreaming"-experiment. Worked great.
I think I tried this method and it did not work as it leaves data stranded in the fifo if it was during an idle interrupt when you set RTS in software. You will have to add code in read function to drain the fifo, which then makes the change not simple anymore.