

Nope, from how Google puts it, play services will block any and all apps without a valid signature from working at all on play certified devices.
You’ll be able to install them via adb probably, but when you run them play services will stop you.
Play services already prevents certain apps from starting until you say a yes/no on a popup, so the framework for this is already in place.
You’ll be able to install them, but play services will very probably stop them from starting.
They already have the framework for this in place, “unrecognised” apps get blocked by Play services already asking you if Google can scan the app before you start it, the app will not start at all unless you click yes or no.