scale and/or distance (resolution) are 2 common fields in many iso19139 oriented metadata profiles, such as https://geonovum.github.io/Metadata-ISO19115/#resolutie. I suggest to include these properties in either the spatial or content section of pygeometa, and return them as part of the iso19139 generation in the relevant fields

identificationInfo[1]//spatialResolution//equivalentScale//denominator and identificationInfo[1]//spatialResolution/*/distance