At the current status, the 3Dconnexion controllers cannot be used with SketchUp Free.
However using their binary drivers is not the only way to use the space mouse, and it is (in my opinion) not even a favorable way, anyways. The 3D mouse is actually a quite standard gamepad device which can work without specialized drivers using the standard Gamepad API of modern web browsers. This is totally portable, cross-platform and without installing any extras and won’t break every new year. We won’t have to wait until the SketchUp team figures out how to make not only Ruby plugins work in SketchUp Free but also binary plugins (which I believe won’t ever happen). Instead, they could just use modern web APIs.
I did this some time back in about 1h and am using space navigator input for an immersive 3d app.
http://www.babylonjs-playground.com/indexstable#B6WKWB