When building a pillar of what seems to be any material (I've only tested cobblestone, stone, and obsidian) on the top of the end portal "location" the dragon will go to the top of whatever height the pillar is. However, the dragon will also consequently delete a few blocks below him, causing the pillar to be whittled down. If you use Obsidian, since the dragon cannot destroy it, the dragon returns to the new height multiple times, instead of destroying it every pass. Included are screenshots of the dragon on the pillar.