A lad in our office has a YRV 4trak and a quick walk across the car park proved that nope - the 4wd version also has drums on the back.
Also reading up on rear disc conversions people have tried it in the past using the Yaris T-Sport discs and only ran into mega bias problems and had to revert back.
I am talking to Dan about beefing up the fronts nicely though .