C is E and E is C in the 3D interpreter.  Only one axis is mapped (C) to motors.  You use E to control it incrementally, if needed, regardless of G90/G91 setting (that WILL affect C).
Yes, it will all be custom LUA scripts.  Because there really isn't anything standard in the 3D printer world.  It is forming up a little, but not there at all yet.  
Steve