Please use the user list (so you can connect with more MongoDB users). The developer list is used for folks working on the next release of GeoServer.
In this case you may wish to look at the GeoTools codebase where the MongoDB support is implemented: https://github.com/geotools/geotools/tree/master/modules/plugin/mongodb
And reach out to the geotools user list.
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Jody Garnett