A good way might be to make using alternative DBs so compelling that people want to use them and end up making adapters 
For me personally the only two DBs I have thought about using other than Postgres is either a GraphDB like Neo4J or something to use with Nerves like Sqlite. I think Regina was working on an Ecto Neo4J adaptor and there is an Sqlite adapter by @scouten (so I’m guessing may be at use at Adobe).
Was there any DB in particular you had in mind? Are there compelling reasons to use it other than PG?
Another way might to perhaps start a marketplace where companies who need adapters (or any kind of Hex package really) pay to have one created.
Another option may be to approach DB teams themselves and ask if they would be willing to make (or assist in making) an Elixir adapter. You often find such teams have their own favourites when it comes to languages (or language types) so it could just be a question of contacting someone on the team who is excited about the idea 
especially if you can demonstrate the community would be highly receptive to using their DB with that language.
