I have been trying to setup cloud logging so that logs are written to cloud storage but excluded from StackDriver logs. Reading the documentation I think this is possible and that exports should get priority over exclusions.
On the log router page I have created a sink to write logs to cloud storage, this is working on its own and I can see the logs in the selected bucket.
From the "logs ingestion" page on the "Exclusions" tab I defined an exclusion. But once this is enabled logs are no-longer written to the bucket.