That functionality was removed in favor of OAuth. I'm not sure M66B needs to get approval for the other builds, though, because the access is gated at the clou

That functionality was removed in favor of OAuth. I'm not sure M66B needs to get approval for the other builds, though, because the access is gated at the cloud API, not though client libraries. You can use Play Services to grant OAuth tokens, or you can use the boring old Google API client libraries, or roll your own; you just need to add the other signing key fingerprints and application IDs to the credential in the project's cloud console. I could easily be mistaken, but there are numerous open-source projects acting as mail clients through the GMail API, Google has granted them access, and they don't have to use a closed-source client to do it. Most of them don't even target Android.