Generally I think maintenance dosing is done orally rather than injected, for slower absorption. I think that the idea of methadone rather than heroin for long term maintenance is that it’s more difficult to get high on, and the high is not as much fun
What you’re describing actually sounds a lot like the way methadone / Suboxone clinics already work though? Even in the US, methadone can be legally prescribed for treatment of opioid addiction, and Suboxone is available in some places - I seem to recall that California was the first state to make it available
