Have you ever considered giving them a warm mash of veggies or fruit instead? Seems to me like baby formula is more of a sustaining foodsource for babies and not so much a "treat" for adults (calorie content, etc.). At least once or twice per week I heat up a sweet potato and puree' it in my food processor with some of his favorite veggies--broccoli, snap peas, and some carrots.
Hopefully someone here can comment more to the effect of how handfeeding ADULT greys could or couldn not impact the bond between human and bird. Sounds like your greys are independent adults, and would naturally not be as "clingy" as they were as babies. Not that they don't love you, but they're just grown ups.
Chimay and I have special time set aside every day to play together...for at least 1/2 an hour when I get home we play on either his basket, or I put a sheet on the coffee table and we spend time together with no distractons (TV, radio, phone, etc). If it's bonding you want, I'm not sure that it can be accomplished SOLEY by treats.