Hi boatstalker - I'm not familiar with YafRay, but I believe the others will only work on their own types of files and not common file types like OBJ.

For Indigo, you can use Blender as an in-between bridge - export an obj file from MoI then bring it into Blender, and then use the Indigo exporter from Blender, here is some info: http://www.indigorenderer.com/blender_tut/

I believe that Kerkythea is another free stand-alone renderer that does support importing OBJ files, so that one is probably worth checking out, it might be less steps to use it: http://www.kerkythea.net/

Maybe others may have some advice...

- Michael