Use the externalCacheable attribute of the ProductCatalog repository to identify transient properties for caching. For example:

<property readable="true" queryable="false"
   display-name-resource="length"
   expert="true" hidden="false"
   cache-mode="inherit" name="length"
   data-type="long"
   property-type="ExampleDescriptor">
…
     <attribute name="propertySortPriority" value="-8"/>
     <attribute name="externalCacheable" value="true"/>
</property>