Screenspace is a free Maya plugin by Animal Logic software engineer Eddie Hoyle that lets users add pickable shapes to transforms that behave as if attached to the viewport. You can basically create rig controls that function as a custom HUD.

The tool lets you assign simple geometric shapes to transforms, then position them at any point in the Maya viewport.

The objects can then be selected by clicking or dragging over them (like a custom heads-up display). For example, you can create facial pickers for character rigs or use it for hard-surface rigging.

Screenspace is free (Apache 2.0 licence) for Maya on Windows, macOS, and Linux. You will have to build the plugin directly from the source code (you can find a guide on GitHub).