Acceptable reason for contacting CPS?

Maybe. And I know that's not the answer you wanted to hear, but these things can be dependent upon stuff like precedent in the area and even how well-staffed (and competently staffed) a CPS office is.

If you want to get involved and pick up custody, then contact a family lawyer and work with him or her about this. They may have other ideas and another way of approaching it. Because if you approach it vindictively, then CPS is likely not going to be too thrilled with enabling an ongoing in-family fight.