The RDS add-on is kind of a funny case that predates the official API. With the API that's in place now, providers should never need this functionality. The patterns for configuring add-ons are:
1) Set environment variables during the provisioning call. This is what 99% of all providers do, and what the RDS add-on would do if it were a real add-on.
2) Implement SSO to your service dashboard. If there's additional configuration or setup that can be done, it should be done in whatever web based control panel you have for your service already.
Best,
Morten