Since no one had responded yet, I went ahead and called Wallet support, which transferred me to the Money Network mobile purchases team (or whatever), and they confirmed that the Google Prepaid Card cannot be used for Google Play purchases.
I didn't ask specifics, so maybe she was only referring to this preloaded cash that Google provided, but looking at my account (only the preloaded Prepaid Card; no additional cards tied to the account), she determined that I couldn't use it to buy apps. I assume there must be some way you can use the Google Prepaid Card to make purchases from the store; otherwise, anyone without a Citi Mastercard or direct carrier billing would be out of luck, no? In any event, I was told that I couldn't.