Please see my blog to read why I created Version 99 Does Not Exist and its predecessor no-commons-logging.
Version 99 Does Not Exist emulates a Maven 2 repository and serves empty jars for any valid package that has version number 99.0-does-not-exist. It also generates poms, metadata files (removed since 2.0) and of course the appropriate hashes.
For example the following links will give an empty jar, its pom and the maven metadata for commons-logging.
Vesion 99 Does Not Exist source code on GitHub.