Printrboard Rev. D - RX1 and TX1 (second serial port) / Marlin
I was able to resolve this issue: 


I moved to Marlin 1.1.9 (the final release of the 1.1.x versions).
I made a fork, for those who would like to see the changes. Also summarized below.
The RX1 and TX1 pins are well identified in the question. I connected those directly to the HC-06 bluetooth module (using a voltage divider for the RX pin in the bluetooth, as it is a 3.3 V logic module).


Changes in code: 

Marlin/Conditionals.LCD.h: commented out: 

// this line conflicted with the bluetooth/serial1 options
// #define USE_MARLINSERIAL !(defined(__AVR__) &amp;&amp; defined(USBCON))


The rest are the configuration options already provided by Marlin in Marlin.Configuration.h: 

define SERIAL_PORT 1
define BAUDRATE 115200  // Note: this baudrate MUST match the one set in the bluetooth module. That's done using the AT commands. There is plenty of information on how to do that. 
define BLUETOOTH


As a result of these changes I am able to print with my PrintrBot Simple 2014 (Printrboard Rev. D) with Bluetooth over a distance of ~3.5mts, behind a wall. I haven't tested it exhaustely, but it does print.