In order for Codehaus to provide useful metrics on downloads, we have to classify each of your artifacts into a version.
You can see a list of all artifacts in a project's Versions (e.g. Groovy) section.
This functionality is for non M2 artifacts. M2 has its own weird and wonderful management system and we have no interest in getting involved with it.
Xircles (in theory) supports a remote API.
The Remote API page contains more information on this API.