.. with software such as Blender...
You want a 3D model of the kart? That should be an issue on development and probably lays somewhere within the realm of #58. Using the favicon may not be the right choice... but not sure what that site you mentioned does exactly. You are welcome to reference it here to development too.
Getting the GM contributor for their mascot to do 3D may not be successful when the 2D svg support was requested and fizzled.
Last year, I have suggested in the SuperTuxKart forum to add a monkey figure and kart that would represent Greasemonkey and OpenUserJs.
Is there anyone here who designs 3D models with software such as Blender, and wants to provide a monkey figure with an OpenUserJs branded kart?