That sounds rough. I'm in Santa Cruz, CA, and I'm not sure how the weather quite compares, but I wouldn't really want sandals. They make some pretty fatty wool socks, but I'd think the lack of any sort of vapor/wind barrier would result in a lot of cold. You could get a waterproof breathable over sock. Then again, very nice socks and oversocks and such aren't super cheap, so I wouldn't dismiss the idea of a cycling shoe. The cheap varient would of course just be a bag or something, but it won't breathe.

Personally I've been rocking neoprene overshoes on cycling shoes recently on colder rides. I did a ride in December where I couldn't feel my feet for hours, and couldn't warm them at all on the side of the road, so now I'm paranoid.