It's too late for me to use now, but the synthetic Polyfilla looks a good idea. Adding mortar is one of those skinning a rabbit jobs and I am always keen to read other modellers techniques.
Like others, I found diluted enamel did not work on laser cut brick courses because it soaked upwards and downwards into the wood courses and all-but disappeared after a few hours. I have used undiluted enamel for the past couple of years. After an hour, the bricks were wiped clean with turps on a tissue and then the brick were dry-brushed at an angle with the original colour. Muck & water runs were added with much diluted PPC 'dirty black'.