Hey trhunter,
this is possible, indeed, but it isn't as intuitive as in Substance Painter right now.
To view a single output in Substance Designer, you need to turn off all those you don't want to see.
You can do that by clicking on Materials -> Default -> Channels in the 3D view toolbar.
Turn off the channels you do not want to see. You can keep those turned on you are not using anyway, of course.
Best Regards