.obj model files are just ASCII. Each line describes either a vertex (as a set of 3 floating point numbers, as coordinates in 3D space), a face (3 vertex references), or slightly more exotic things, like texture coordinates, or vertex normals. You can easily write a script to read those sorts of files and put the relevant information into an object (in this case, I call them Virtual Meshes).
The virtual meshes have a position, the vertex coordinates, which vertex groups form faces, and some light shading calculations (so the thing isn't one big blob of one color) that involve calculating normals and some basic vector operations to calculate available light without self occlusion.
Once all of that data is available, it is just a matter of issuing commands to draw triangles. This requires a small change to the PlayGUI object, but that is something accomplished purely with script modifications.
No machine code level modifications, if that is what you are asking.
What kind of "mod" needs to be applied to the PlayGUI object?
It's type needs to be changed to the editor's GUI type. It behaves essentially the same way, except you get the method calls for drawing lines, spheres, and triangles. You also lose the ability to see beacons (except around the edges of the screen), zoom, and several HUD elements become more transparent.
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What kind of "mod" needs to be applied to the PlayGUI object?
Edit:
I tried changing "new GameTSControl" to "new GuiControl" and now I can't see anything.
This is what I see: