This could be done fairly easily, perhaps more easily, as a standalone program, i think. Something that exposes a MongoDB-compatible API, and translates requests into libpq to go to a PostgreSQL backend.
You could run it either on the PG host, or on the same host as the apps wanting to talk to PG.
Based on a quick look at the documentation, i think the API Waterline exposes to clients is not an imitation of the MongoDB API, which would allow PostgreSQL to be used as a drop-in replacement for MongoDB. Rather, it's its own thing.
Waterline does support both MongoDB and PostgreSQL, though, so you could use it as part of a migration strategy: start with MongoDB, install Waterline backed by MongoDB, migrate your app to use Waterline rather than using MongoDB directly, replace MongoDB with PostgreSQL.
You could run it either on the PG host, or on the same host as the apps wanting to talk to PG.