How does one use the new Center Attachment point?

[Nalates Urriah]

We have two new attachment points; neck and center. Using the neck point is obvious. It previously could be animated, so it works like other attachment points. I expect a change in the 'wear' menus to add the point.

The neck point is also useable for rigging mesh.

The center point apparently does not animate. If the avatar is dancing, anything attached to the center point would remain stationary.

Does Center affect rigged mesh?

How does one add it to the skeleton for animation and rigging in Blender?

EDIT--------------

Viewers with the new points are SL Dev, Dolphin, and Exodus. More info: #SL’s New Attachment Points

[Ceera Murakami]

When did these get introduced, and what viewer supports them? I don't see these as existing in Viewer 3 (current release version) or in the Development version of Viewer 3.