Default Value for Bidirectional differential signal

I am a bit unsure what actually you want or why. Unused pins (all) have options such as (as inputs tristated, ...etc) and you can choose whichever. But it has to be unused, so no logic connected to it.

If you want unused pins to be other than that but you only want those pins to be tristated then you can apply 'Z' permanently.

If you want to keep it bidirectional and I don't see why you can use a dummy select input for output enable, I mean use an unused input to do that selection and fool the tool