I use quinoa quite often, but although it's not technically a grain, it has the mouthfeel of a grain so I'm not sure I'd pair it with potatoes. I have used beans with quinoa, but the duckfat is so luxe that it might overpower the beans. I think I'd go with an animal protein with this combo (I'm guessing that once you're using duckfat, there is no vegetarian issue).

The brussels sprouts, potatoes and duck fat sound fine by they ownselves. Why mess with it? You can subtract and rearrange but combined the don't sound very good at all as a package.

I would roast the vegetables until almost done, then add in the cooked grains, then toss back into the oven to combine for the last 10 or 15 minutes. You don't want the grains to dry out, but still want the flavors to meld.

I've use canned rinsed chick peas as an addition to roasted vegetables and they work out nicely.