Alright, figured it out. We're running Casbah/Salat on the same app, for now. There's a mongodb.uri in the config file that gets read in by both. However, ReactiveMongo seems to only work if the database name is included, which according to the mongodb "connection string uri" spec:
http://docs.mongodb.org/manual/reference/connection-string/
... you only need to include the database if you have credentials you need to authenticate with. In our case, we don't have credentials, so Casbah wasn't including the database. I added it in anyway... casbah ignored it safely, and reactivemongo worked. I neglected to do the same in the test config file, so even though it was showing the correct host, it wasn't about to work correctly.
I see how the host url + db name in one string replaces the two fields "mongodb.servers" and "mongodb.db", but it can be confusing if not conforming to mongo's similar spec.