We should be able to specify a custom maven wagon for downloading artifacts. Apparently, you can publish with a custom wagon, but not download. (See http://forums.gradle.org/gradle/topics/gradle_ignores_custom_maven_repository_ftp and http://forums.gradle.org/gradle/topics/use_custom_protocol_transport_for_gradle_artifact_repo)
There’s a lot of custom wagons for maven that are pretty useful, particularly for resolving artifacts on s3.