It has always seemed to me that when you combine derailleurs with an internally-geared hub (IGH), you get the worst of both worlds. Derailleurs are vulnerable to bending and breaking and need more frequent maintenance. IGHs are heavy and expensive. You may have a plan to mitigate the cost. Rigging up these two cogs might be a technical challenge, but that can be fun and rewarding. If you want it to be durable, I suggest you get one of those ugly derailleur guards that prevent the bending-in of the derailleur/hanger. They work, and you want a heavy duty bike, right?

Another consideration is that IGHs are not rated for the high torque that you make when you gear them very low. You can probably get away with it, though.