Preheat the oven to 450 degrees F. Cut the hens in half with kitchen shears, cutting along one side of the breastbone. Place the hens on a rimmed baking sheet, cut-side down; season with salt and pepper. Scatter the lemon slices over and around the hens. Roast until the hens are golden, about 20 minutes.
Meanwhile, melt the butter in a medium saucepan over medium-high heat. Add the onion and a pinch of salt. Cook, stirring occasionally, until softened, 3 minutes. Add the couscous and cook, stirring occasionally, until lightly toasted, 3 minutes. Add the broth; season with salt and pepper. Bring to a simmer, then reduce the heat to low. Cover and cook until most of the liquid is absorbed, about 8 minutes. Stir in the zucchini, cover and cook until tender, about 3 more minutes; set aside.
Combine the honey, cumin, paprika and lemon juice. Brush on the hens and continue roasting until cooked through, 5 to 7 more minutes.
Stir the cilantro and lemon zest into the couscous and season with salt and pepper. Serve with the hens and drizzle with any pan juices.