1. In a blender or food processor, combine dried cherries, oregano, cumin, paprika, oil, and lemon juice. Whirl until pureed, scraping container sides as necessary. Add salt to taste.
2. In a 1 1/2- to 2-quart pan, combine couscous and broth. Bring to a boil over high heat, cover, remove pan from heat, and let stand until liquid is absorbed, about 5 minutes. Keep warm.
3. Meanwhile, rinse halibut, pat dry, and coat with cherry mixture. Set each piece of halibut on a slightly larger piece of foil. Cut foil to fit fish.
4. Set foil with fish on a barbecue grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds). Cover barbecue with lid (open vents for charcoal), and cook until fish is opaque but moist-looking in thickest part (cut to test), 8 to 10 minutes.
5. Stir chopped cucumber and chopped mint into couscous; spoon onto plates. Cut grilled halibut into equal portions and set onto couscous. Season to taste with salt.