ModeShape 3 distinguishes between content owned by ModeShape vs content that is in an external system and owned by that external system. All content owned by ModeShape will be stored in ModeShape's internal representation (one or more BSON documents for each node), and this content is merely read from or written to Infinispan (and thus its CacheLoader implementations).
To talk with external systems, implementing a CacheLoader would likely work, but it might not be very practical. For example, there's no path information in the internal representations, so working with a path-based external system (rather than a key-based one) would be quite a challenge.
Therefore, ModeShape 3 should define a new SPI that can be easily implemented to read or write nodes, properties and children from/to an external system. It should provide a way to project those external systems into the repository content (preferably configured or defined via content rather than via configuration).
See the discussion thread for the various options and requirements.