Will just have to figure out what we want the user interface to be like; should it be a separate guc, or somehow cram it into wal_level?
Yeah, I had brought up similar idea up thread. Right now wal_level is nicely ordered. But with this additional logic, I am not sure if we would need multiple new levels and also break that ordering (I don't know if its important). For example, one may want to set up streaming replication with/without this feature or hot standby with/without the feature. I don't have a good idea about how to capture them in wal_level. May be something like: minimal, archive, archive_with_this_new_feature, hot_standby and hot_standby_with_this_new_feature.
