Not sure if this is what you are looking for, but we send serial debug traces over the same serial port the kernel uses for debug spew--the connector at the edge of the board--using code like this:
RingBuffer rx_buffer_dbg;
TTYUARTClass console(&rx_buffer_dbg, 3, false); // Linux debug port
...
void setup()
{
delay(10000); // wait for boot to complete...!
console.begin(115200);
...
console.println("Hello, world!");
...
}