I was intrigued by a question in the help forum about building buildings on space terrain. I wasn't able to build one, but I found that you can land a Terran building on space terrain quite easily. All you need is to use any black tile from a tile group that contains buildable tiles (for example, [4-6]). Map is attached.
As a side note, DrunkenWrestler was able to extend creep over normally non-walkable terrain and build things on it, but that was in the badlands tileset. I don't know if any similar tiles exist in other tilesets, but I would assume they do. This only works for zerg buildings, of course.
The creep extension can be used over any null terrain I've found. Then units can walk off onto what ever unwalkable surface you have. I used this in an experiment to put as many units into a single space as possible, but thats another story.