I've been watching a handful of this extended campaign type for a while now and I just saw a video where someone created an Ork war-band by using a dice-roll sheet prepared from custom rules. Can anyone tell me where to find such custom rules, or how to create a dice-roll sheet for a specific faction? I find Path to Glory more in-depth than the core game modes, but I only have regular space marines.
Mini War Gaming published a series of battle reports of Chaos vs. Orks.
You can find them here (half of them are free to view, the rest need a subscription).
While these aren't the same exact rules used in Mini War Gameing's videos (I think), they look like what you're looking for.
I guess you can also try to ask MWG on their forum to share these rules, or make your own version using the videos and the PDF as a starting point...