Space Navigator is used by about half a dozen users around here, it's just not a priority.
Please do not underrate that! Look at the Forum >there are a lot more then a half dozen who guess the SpaceNavigater would be a greate expantion to Unity3D.
Especialy is there no arrgument against to link the SpaceNavigator as an input device for common 3D game navigations. (As it can be done with a Joystick)
Look at the screenshot. It shows, that SpaceNavigator sends coordinate datas like an other Joystick too.
Is it that heavy for you to implement this to use it like an Joystick?