Is there a way to Query what's "NOT THERE"?

What should be done, if you don't already have a larger study area polygon, is to create one. Then, clip/erase using the other tenure data the larger polygon to create the smaller polygons of your unused land.

As @Emily was inquiring about, if you have a bounding polygon or as you call a region you can use this layer to come up with the difference to create the Crown Land polygon layer. To do this you could use the Erase tool to erase away your region layer against the land tenure features leaving the Crown Land layer.

The only way I can think of is to create an averlaying single polygon (which covers all the area of interest).
Then perhaps the erase command using the big polygon as input and the smaller polygons for the erase layer.