In a small bowl, combine the salt, garlic powder, cayenne, thyme and pepper. Rub 1 tablespoon of the olive oil over the Cornish hens and rub with the spice mix. Transfer the hens to a resealable plastic bag and let stand for 20 minutes.
Preheat the oven to 400°. In a large ovenproof skillet, heat the remaining 3 tablespoons of olive oil over moderately high heat. Remove the hens from the plastic bag and add them to the skillet, skin-side down. Cook until browned on the bottom, about 10 minutes. Using tongs, turn the Cornish hens skin side up. Transfer the skillet to the oven and roast the hens for 12 minutes, until cooked through.
In a mini food processor, combine the citrus juices with the garlic and red pepper and pulse for 30 seconds. Season with salt and pepper.
Heat the broiler. Broil the hens 6 inches from the heat for about 2 minutes, until the skin is crisp. Transfer the hens to a work surface and halve them lengthwise; set the halves on plates.
Drain all but 2 tablespoons of fat from the skillet and set the skillet over moderately high heat. Add the mojo and cook, scraping up any browned bits on the bottom of the skillet, until the sauce is thickened slightly, about 2 minutes. Pour the mojo over the Cornish hens and serve.