This is definitely a cool idea.
One thing I've noticed with games that let you do this is that the weapons you can attach to things end up being terribly generic and uninteresting. You mentioned Blockstorm as an example. Is that a fun game? Other recommendations?
Voxel technology certainly lends itself well to this sort of thing, so it's something I'll think more about.
If there was ever a multiplayer mode, being able to customize your avatar would be reeeeeeaaally neat, allthough I suspect half of all robots would suddenly have/be genitals... This tends to be a thing with user-generated content. :)