Personally, I'm not a fan of clipless in snowy weather. Pinned platforms work pretty well and you have all sorts of footwear options. That said, I've heard of the sandal approach and my guess is that the same rules apply as for any other part of your body:

- a base layer to wick moisture
- an insulating layer
- a wind blocking layer.

If you can get the above inside your sandal I don't see why it wouldn't work. I think the only thing to worry about is if at any point during the pedal stroke the sandal is compressing the insulation layer. That's pretty much a given at the bottom of your foot so you'd want to get some degree of insulation from the sole of the sandal itself.