I2C with only ground common
Should be fine,if the voltage on the pullup is at an acceptable on for the slave and the master.  If not, you need a level shifter.  Also, you need to pay attention to total cable capacitance if you want to keep your speed up.  You might need to use smaller resistors than you expect.

http://www.edn.com/design/analog/4371297/Design-calculations-for-robust-I2C-communications seems like a good reference.