I already use Readers (ESRI Legacy) to "merge" tables based on a relation condition. This can be done by adding the SDE30_IDs clause to the mapping file. (Since it's not supported in Workbench)
Since Readers and FeatureReaders seems to use the same underlying libraries, would it be possible to do the same thing at the FeatureReader level. This would allow flexible and efficient way to retrieve data since a more complex conditional clause could be built along the transformers way.
Thanks,
Mathieu