The new internal channel option is now available in the Google Play Deploy step v1.4.2! 
Sorry for the late reply @j.kaan, our devs just got excited and went ahead to add it themselves, hope you don’t mind 
P.S.: For step dev you can find the getting started at: https://devcenter.bitrise.io/bitrise-cli/create-your-own-step/ and infos in the https://devcenter.bitrise.io/bitrise-cli/ category, as well as in the steps’ README. If you’d have any questions just let us know!