I know this isn't exactly answering your question, but if you deploy on an Amazon EC2 instance, you can use MongoHQ's shared hosting in conjunction to your pre-existing Node provider. MongoHQ runs on the AWS infrastructure, so the latency between your Node server and MongoHQ is insignificant (we use EngineYard with MongoHQ and the latency for a simple small read is anywhere between 1-5ms).

Make sure that the regions match up though! :D