Finally got a reply:
Thank you for your interest and enthusiasm in writing for Games Workshop.
Unfortunately, your submission is not right for us at this time, and therefore we will not be taking your current application any further. We fully appreciate that you may want feedback, however due to the volume of applications we are not able to provide any.
We wish you every success in your future writing endeavours.
The Black Library Author Team
Looks like I need to spend more time writing to sharpen my skills some more so that next year I'll have a better shot at it.
I'm tossing out the work I had on the story since it was mainly an exercise to try and put the idea into a more solid form than a story proper.