In lieu of a dedicated refrigerator for fermenting, I think a swamp cooler is the best way to go for you. Basically a large tub with water that will hold your fermenter and frozen water bottles to control the temp. As said earlier, you will have to swap them out quite regularly to keep your temps stable. I would probably just keep the AC running. Won't you have a room with the AC running all the time anyway?
I don't keep the A/C running when I am not home, or if I am not spending time in that particualr room (each room has its own A/C, it is not one central unit). My guest bedroom is the smallest unit/room and would therefore use the least electricity, so I could keep it in there with the A/C on constantly until I work out a permanant solution down the road...
The swamp bucket sounds decent but also a lot of work, especially since I am not home on many weekends. I would also think this method would be difficult to maintain a steady temperature. How important is a steady temperature anyway? Is it bad if it fluctuates more than 3 or 4 degrees in either direction over the course of the day?