@mfalkvidd I'm using an AVR board for my serial gateway. I modified hwDebugPrint in MyHwAVR.cpp with an extra #ifndef to look for MY_DEBUGDEVICE or copy MY_SERIALDEVICE to it. Then all occurrences of MY_SERIALDEVICE were changed to MY_DEBUGDEVICE.
I added this to my sketch:
#define MY_DEBUGDEVICE Serial  // Redirect debug to Serial
The result was what I wanted. Controller <-> Gateway configuration went over Serial1 (/dev/ttyS0 on my RPi) and debugging output went to Serial (/dev/ttyACM0 on my RPi). This allows me to easily debug without having to take any special steps with my controller.
void hwDebugPrint(const char *fmt, ... )
{
#ifndef MY_DEBUGDEVICE
#define MY_DEBUGDEVICE MY_SERIALDEVICE
#endif
#ifndef MY_DISABLED_SERIAL
	char fmtBuffer[MY_SERIAL_OUTPUT_SIZE];
#ifdef MY_GATEWAY_SERIAL
	// prepend debug message to be handled correctly by controller (C_INTERNAL, I_LOG_MESSAGE)
	snprintf_P(fmtBuffer, sizeof(fmtBuffer), PSTR("0;255;%" PRIu8 ";0;%" PRIu8 ";%" PRIu32 " "),
	           C_INTERNAL, I_LOG_MESSAGE, hwMillis());
	MY_DEBUGDEVICE.print(fmtBuffer);
#else
	// prepend timestamp
	MY_DEBUGDEVICE.print(hwMillis());
	MY_DEBUGDEVICE.print(F(" "));
#endif
	va_list args;
	va_start (args, fmt );
	vsnprintf_P(fmtBuffer, sizeof(fmtBuffer), fmt, args);
#ifdef MY_GATEWAY_SERIAL
	// Truncate message if this is gateway node
	fmtBuffer[sizeof(fmtBuffer) - 2] = '\n';
	fmtBuffer[sizeof(fmtBuffer) - 1] = '\0';
#endif
	va_end (args);
	MY_DEBUGDEVICE.print(fmtBuffer);
	MY_DEBUGDEVICE.flush();
#else
	(void)fmt;
#endif
}