I think the actual definition of the Serial object happens in HarwareSerial.cpp
Here:
// Preinstantiate Objects //////////////////////////////////////////////////////
#if defined(UBRRH) && defined(UBRRL)
HardwareSerial Serial(&rx_buffer, &tx_buffer, &UBRRH, &UBRRL, &UCSRA, &UCSRB, &UCSRC, &UDR, RXEN, TXEN, RXCIE, UDRIE, U2X);
#elif defined(UBRR0H) && defined(UBRR0L)
HardwareSerial Serial(&rx_buffer, &tx_buffer, &UBRR0H, &UBRR0L, &UCSR0A, &UCSR0B, &UCSR0C, &UDR0, RXEN0, TXEN0, RXCIE0, UDRIE0, U2X0);
#elif defined(USBCON)
// do nothing - Serial object and buffers are initialized in CDC code
#else
#error no serial port defined (port 0)
#endif
#if defined(UBRR1H)
HardwareSerial Serial1(&rx_buffer1, &tx_buffer1, &UBRR1H, &UBRR1L, &UCSR1A, &UCSR1B, &UCSR1C, &UDR1, RXEN1, TXEN1, RXCIE1, UDRIE1, U2X1);
#endif
#if defined(UBRR2H)
HardwareSerial Serial2(&rx_buffer2, &tx_buffer2, &UBRR2H, &UBRR2L, &UCSR2A, &UCSR2B, &UCSR2C, &UDR2, RXEN2, TXEN2, RXCIE2, UDRIE2, U2X2);
#endif
#if defined(UBRR3H)
HardwareSerial Serial3(&rx_buffer3, &tx_buffer3, &UBRR3H, &UBRR3L, &UCSR3A, &UCSR3B, &UCSR3C, &UDR3, RXEN3, TXEN3, RXCIE3, UDRIE3, U2X3);
#endif
It appears to decide based on a register or a bit in a register having its name defined. If your chip has that register defined in a different way then this may not line up and create an object for you. HarwareSerial is pretty well documented though, it shouldn't be hard to go through and find the places where you have to make chip dependent decisions and you can create your own instance in your sketch.