The Biped CoM won't let you use the curve editor how you would like. Yes it sucks, but you have to give something up to get ease of use and features in other areas. But they don't leave you totally twisting in the wind, just partially. Instead it uses a TCB Controller, to edit the TCB info for a given key you right click…
Ignore everything I said before I just remembered a simple work around. You'll kick yourself when you read it, I did when I remembered it. Create a dummy object, snap it to the CoM, link the CoM to the Dummy and use the Dummy as the new CoM. Edit the dummies curves as if it was the CoM and you're golden. If it won't let…