Non-JWs can be used as eyewitnesses to establish the "two witness" bit. But as noted above, it's as "witness for the prosecution" only.
The judicial committee is primarily there to cast judgment on the "genuineness" of the repentance of the accused, as if that were somehow possible.
EDIT: Just got to add, I would think that inviting a "worldly" person to a JW judicial committee would be the number one, absolutely fool-proof method to inoculate that "worldly person" from ever joining the JW nuthouse.