The use of anything other than Apple Juice would throw off the taste of this brine. If you don't want to use apple juice because you feel you are wasting it, you could always buy a 20 oz bottle of apple soda (Manzanita Sol is my personal favorite) or find other uses for the apple juice (Freeze them into ice cubes).
I would consider orange or other fruit juice on hand for the spice combination and for cornish , maybe a tin or mandarine oranges and use as a sauce when thickened . Be sure to marinate in refrigerator if doing for overnight or six hours which seems long for Cornish which are very tender young birds
I like to bake an apple pie once in awhile which recipe calls for 12 oz. of apple juice, but I have used a couple of cups of apple sauce and some water and it turns out just fine, but in your case it may not be quite close enough to be right. Good luck.
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