Well, I got Mac to compile, but it's not good. My code works, but the system is doing all weird stuff. If you have the official 3Dconnexion drivers installed, then it takes over control and leaves the device open, meaning I am not able to connect to it. I'm not sure why this is, on Windows it works fine with multiple apps having access (just not at the same time). On Windows you can also disable specific apps or the whole driver itself. But on Mac there are limited options, it is very bare-bones compared to Windows, and people on the forum said the only way was to uninstall completely.
That would be fine, except without the official driver on Mac, the Space Mouse does some sort of mouse cursor emulation by default. So while I can connect to it via HID, and that works, it is also moving the mouse around, rotating the camera, zooming in, etc, wildly. And I'm not sure why that is. But even if I could disable it, most people that have devices like this are using them for 3D modeling work, so it would be too much to ask them to uninstall the driver just for a Godot plug-in.
So I don't know. I spent all day on this, but it's getting late to figure this out. At least Linux and Windows are fully working. I will have to see what I can do about Mac. I don't want to release unless it's cross-platform, especially since I know these devices are popular with designers that are likely on Mac. I saw there was some config XML file the driver uses, so I might be able to blacklist Godot. It's kind of a hack, but at least I have a lead.