If one uses StorageOptions.Builder#setHost with a value such as http://proxy.url.local:8888/somepath, builds the storage client using this StorageOptions, most connections generated by the client use these values (scheme, host, port, path), eg when listing and deleting blobs, but the file upload connection does not:
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/52b727aef88ae76984aa3c02b4d7067e198d34b7/google-cloud-storage/src/main/java/com/google/cloud/storage/spi/v1/HttpStorageRpc.java#L720

I think all connections should follow the host setting as a hostname or as an endpoint setting.