Integrate an external application What I'd like to know is, would it be possible to display the output of an external opengl application inside Blender?



As shown on the image the left part is the standard 3D viewport, and on the right would be the output of an external renderer or game engine that would be displayed inside Blender directly.

A use case for this would be to use Blender as a level editor and seeing the edits being replicated live in the game engine.
For an external render engine, yes. Have a look at cycles, it is actually created as an external render engine made available through a python based addon. Luxrender also utilises this to integrate with blender.

For general opengl drawing I would say no. Blender is developed to read it's own data and draw it to screen. There is the bgl module available in python to allow python scripts to draw as needed, you could get data from another application and draw in blender with this but I would not expect decent performance with anything but a very simple scene.