The system now marks newly imported (changed) data with a status flag, which will enable targeting the relevant subset of data. The implementation is not finished yet, but it's already better than it used to. The user must now manually set the status flag if he wants to reprocess something that has already been processed:

UPDATE osm_<table> SET status=1 WHERE <filter>;

The system sets the flag automatically for new or changed data, but it's never set for existing data that remained unchanged during the import. There should be an option for the user to force setting the status.