Would it be possible to allow the use of textures for obj models rendered in-game?

i.e. if a png file is in included instead of an mtl file, it uses the texture. It would make it possible for me to make a content pack.

Lol
You got really committed on this one. I was wrong about this the whole time.

The obj spec points to a mtl using the "mtllib" directive
The mtl spec points to an image using the "map_Kd" directive

If you are using blender, make sure Path Mode: Copy is set in the obj exporter