Thanks for the response, kowalski!
Please forgive my ignorance, but may I ask why the design principle is that the additional UART instances/buffers are not pre-allocated? Is it to save memory? If so, I would "think" that the MEGA certainly has enough memory to handle them being pre-allocated in the #elif defined(ARDUINO_MEGA) block so they can simply be used like the default uart. After all, multiple hardware UARTs was my primary reason for purchasing a MEGA 
Just curious... Thanks!