Finally this is being implemented. The 2.7 trunk version of GeoNetwork has this capability. It's called "OGC WFS GetFeature harvesting". You can specify the query on your PostGIS and map those fields to metadata using a template.
More info in the Harvesing Documentation:
. OGC WFS GetFeature response Metadata can be present in the tables of a relational databases, which are commonly used by many organisations. Putting an OGC Web Feature Service (WFS) over a relational database will allow metadata to be extracted via standard query mechanisms. This harvesting type allows the user to specify a GetFeature query and map information from the features to fragments of metadata that can be linked or copied into a template to create metadata records.