I want to use hardware flow control for AVR Serial.I think I can do it by inspecting RTS/CTS pins in the transmit/receive interrupt routine, but I can't override the interrupt routine because it is defined in HardwareSerialN.cpp as follows.

#if defined(USART_RX_vect)
  ISR(USART_RX_vect)
#elif defined(USART0_RX_vect)
  ISR(USART0_RX_vect)
#elif defined(USART_RXC_vect)
  ISR(USART_RXC_vect) // ATmega8.
#else.
  #error "Don't know what the Data Received vector is called for Serial."
#endif
  {
    Serial._rx_complete_irq();
  }

If we change this to the following, we can override the interrupts without affecting the existing usage.

ISR( vector, __attribute__((weak)) )
{
  //behavior
}

I hope you'll consider changing this.