Lager fermentation is (relatively) easy to do this way. Get a big laundry tub, put your carboy/bucket in it and fill the tub with cold water. the bigger mass will buffer temperature fluctuations pretty well as long as outside temps aren't swinging wildly or we have a heat wave...if so, you can try throwing frozen water bottles into the tub to keep temps down, but you will really have to nurse it to keep the temp constant.
Lagering will be harder, but not as critical to be absolutely stable (IMHO). you really need to keep the temps down around 35 if you do it in the dead of winter and we have a few REALLY cold weeks here in PA you might be okay, but often we have temps that go into the 40's or 50's for a few days.
I have read of a few folks that will bury their carboy in snow to lager it. the snow should keep it at 32 as it will insulate colder air temps and as long as it is buried in a snowbank and the surrounding snow doesn't melt your beer shouldn't raise above 32-ish. Of course this relies on us having a decent snowfall.
I'd say give it a shot, but you are a bit at the mercy of mother nature. Let us know how it turns out.