Going back many months... There is no need for the camera to control the Arduino. Yes, the Bulb ramp function in ML is cool. I know, because I helped write it! But it is trivial to do the same thing in C using manual shutter mode instead. Again, the data only needs to go one way, from Arduino to camera. Yes, there is a delay. You can calculate it experimentally, it is constant. Then anticipate it in the code.
I take it back, you cannot write the Bulb ramp code on the arduino without access to the exposure data. But there is no need for it if you set the exposure correctly. You only need exposure data to determine how long the shutter is open. You can make some maximum assumptions to avoid knowing this.
Or just ignore it completely! Assuming an exposure of less than 20 seconds, just move the slider slowly without synching.