Rather than getting into all of that, your time here might be better spent here finding DBaaS services that host on the same clouds Appliku deploys to, and partnering Appliku with them to guarantee predictable network-transit peering to the clouds your users are launching on; probably by setting up the install-time UX flow so that that DBaaS choice is made early in the setup flow, and constrains further choices (i.e. user chooses DBaaS X; Appliku knows what AZs it exists in in each cloud, and restricts user to deploying the app to those same AZs in order to ensure free transit.)
In other words, do the backend work Heroku does to add a new DBaaS "add-on" partner, but without reselling/wrapping the resulting service, instead just giving a UX to bind to it.