Arduino Hardware serial library with rts/cts flow control support
I am not aware of any such library. You could try to implement one
yourself, and maybe share it with the community. :-)

If you take this route, you will have to copy the file
HardwareSerial.cpp from the Arduino core, make your
changes there, and have your version replace the one from the core. Here
are some ideas that may help to get you started:


Wire the 'ready' signal from the device to an interrupt-capable pin of
the Arduino.
Inside the interrupt handler:


when 'ready' is deasserted, disable the UDRE interrupt by issuing
cbi(*_ucsrb, UDRIE0)
when ready is asserted, enable it again, but only if the output
buffer is not empty:
if (_tx_buffer_head != _tx_buffer_tail) sbi(*_ucsrb, UDRIE0);

Patch HardwareSerial::write() to enable the interrupt only if
'ready' is asserted.
Note that the UDRE IRQ can be explicitly called
by HardwareSerial::flush()
and by HardwareSerial::write() if interrupts are globally
disabled. You will have to condition this on the 'ready' signal.
Note also that HardwareSerial::write() can write directly to the
UART if the buffer is empty. This also has to be
conditioned on the 'ready' signal.