Well, with DCS I don't see how back-EMF is going to work out, I'll let someone else try to get that working. If you're measuring motor voltage, you could logically wait to trigger for the voltage to start decreasing and then trigger the effect.
I'd run the motor voltage detection using an isolated power module and power the whole system within that framework. Probably an A/D input on that Arduino would be the ticket to detect when you were decelerating.
