Remove the giblet packet (if present) from the cavity of the game hens. Carefully rinse the game hens under cold water. Thoroughly pat dry with paper toweling. Let sit at room temperature for 30 minutes.
Set a rack in the center of the oven and preheat oven to 375º. Make sure that the hens are completely dry: if not, pat dry again with paper toweling. Sprinkle some of the salt and pepper (about 1/8 tsp of each for each bird) into the cavities of the game hens. Stuff 1 lemon quarter, 1/2 shallot, and 2 herb sprigs into each cavity. Mix the remaining salt and pepper into the butter and rub all over the hens. Set hens aside.
Set a large, heavy-bottomed, oven-safe skillet or Dutch oven over medium heat and add the porcini oil (or olive oil). Add the shallots, fresh mushrooms, barley, salt, and pepper. Sauté until the vegetables have softened slightly and the barley begins to give off a nutty, toasted fragrance, 5–7 minutes. Bring the heat to high while stirring and add the wine. Continue stirring until about half of the wine has been absorbed/reduced. Stir in the vegetable broth and rehydrated mushrooms. Remove from the heat.
Set a small rack into the bottom of the pot (see Recipe Note #4). Place the hens on top of the rack (balance them against the sides of the pot, if necessary).
Place the pot on the oven's middle rack and roast for 50 minutes, turning the pot 180º (a half turn) once during the roast.
Check the temperature of the hens by inserting an instant-read thermometer into the meatiest part of the thigh: the final temperature must reach 165º. Continue roasting as needed. Also take note of the degree of browning: if the hens are browning too much underneath the herbs, cover them with a piece of foil. If they are not browning enough, remove the herb sprigs and turn the broiler on high for the last ~5 minutes of cooking (estimate based on the temperature of the thigh meat). Watch carefully to prevent burning and give the pot a half-turn once during the broil.
Remove the hens to a cutting board and rest for 10 minutes. Remove the rack from the pot and fluff the mushroom-barley pilaf with a rice paddle or fork. Return the game hens to the pot, if you'd like, and serve garnished with additional herb sprigs.