Control.Tag in GoObject?

Is there anything like winforms’ Control.Tag in GoObject that can be used to store data that’s associated with the GoObject?

No, there isn’t, but for classes that implement IGoGraphPart, there are UserFlags and UserObject properties that you can set. That would be for all GoNodes, GoPorts, and GoLinks.
We didn’t want to have to require every object class to support such properties: partly to save space, partly for design cleanliness.