I have seen that it seems possible to have an Android app display full screen, for example the SmugMug photo hosting site app can display photos full screen making the onscreen navigation bar disappear. To get it back you just need to touch the screen.
If the navigation display did this it would avoid wasting space on the navigation bar, particularly when in landscape mode. This is not too much of a problem on phones where it is possible to lock the home screen such that the navigation bar stays in the portrait mode location which is on one side of the screen.
However, I have been experimenting with using a tablet to run the navigation app, and found that this does not seem to be possible for tablets, where the navigation bar will always move to the bottom of the display and takes up a relatively large proportion of the available vertical space.
I know that I can mount the display in portrait (vertical) mode, but I find this is not as easy to find space for and is also less aesthetically pleasing on the bike, so I much prefer to mount the device in landscape (horizontal) mode, in which case it would be good the get back the space used by the navigation bar.