I guess this is a question of how usefuly, deployable the prototype shoukd be. Why not just have an in memory object cache, literally a hashmap, for your dal? If you're composing app level code, you don't need to know what the backend does to your data. You could even create a simple method to populate the data at app boot in your in the dev profile. When you figure out the storage requirements and finalized model, build your db.

This would save you time on picking the db, schema changes or even migration changes in Mongo. You don't have to worry about bad documents from an earlier app revision.