I have both an I2C display and an SPI display running over digital (25m cables) together with a 1-wire chain, and a collection of analog sensors on A0-A5 also over long cables. I have not tested I2C via the analog pins but see no reason why that would not work.
Generally analog devices perform correctly even over long cables (50m+) but some recalibration is needed (in software) if the readings need to be accurate. The voltage drop over 100m CAT5 is around 0.125v, slightly less over CAT6 so this should be taken into account.
The power provided to the CATkit also powers the Arduino and Kittens (800mA) so the PSU should be 1000mA or better. I have found that a weak PSU affects the I2C more than other protocols.
Thanks for the suggestions Rob! Very helpful!