VeroMartin:
I am working on the same problem, while creating a intervalometer with an arduino is pretty straight forward, the functionality of the bulb ramping of Magic Lantern is worth letting the camera control the slider. I'm using a belt drive stepper motor to move my slider and this seems to produce some vibration, so I'm going to use the ML bulb ramping function to do a Move Shoot Move arrangement. This will minimize image blur since the shutter is only open when the camera is stationary. Does anyone have a pin out of the hot shoe for a Canon EOS 60d? Are you sure the pin-outs are optoisolated from the camera. I don't care that much about my Arduino board as long as it doesn't kill my camera.
I'm so confidence on my measurements as I can be.
I am going to use the 5 volt on center pin, and shorted to ground as a "hold" signal.
100% NO. but confidence enough to try it on my 550d, YES 
I have a TF-326 Flash Hot Shoe to PC Sync Adapter for CANON, and going to use it to an analog input on the arduino, had some problems when it was connected on a digital pin on my test rig (it could not sense the "open" circuit. so it only got one "hold" when camera was triggered.)
I got my stand alone arduino and a usb programmer a while ago, but i cant make the usb programmer do its work, so i have ordered a standard uno, which i steal the ship from, and use the arduino board as a programmer instead
I hope you manage to do what you want it to, and please feel free to ask.
I'm sorry for late updates, just finished school, and been trolling for a decent work lately, but these Monday, MAGIC 
I got a job
I think we are on the same page on these project