I am currently on HCG as well and am wondering the same thing. I've heard many people on the HCG forums state that the 500 calories allowed are mostly to keep us from "protein deficiency", but fasting is not starvation so I don't believe this deficiency would occur...? I'm no expert though.
Hopefully, someone else will chime in, otherwise, I guess it would be best to just experiment with it for a few days and see how your body reacts. 22 days is wonderful. How much have you lost and how long do you plan to go for?