I dont own a OWB, but I know a few who do. I'd think if your lines are five feet deep like you say, the chances of freezing are almost nill. Normally, frost doesnt penetrate five feet in undisturbed areas. Under driveways and other traveled surfaces the frost can reach that depth. Ground cover on top of your lines makes a big difference also in added insulation. Also the heat circulating throughout the lines all winter will warm the ground keeping the frost from reaching too deep. And if power was interrupted for a few days or a week in a severe instance, the frost most likely wouldnt travel down to your lines that fast anyway.
I run heavy equiptment for a living here in central Mn. And we do plenty of frost ripping in the winter. Its very rare where I have seen a full 5 feet of frost. But I have, mostly on traveled and unexposed areas. Ground cover makes a huge difference, you would be surprised what difference a few weeds or grass. I'd have to assume your lines are inside some kind of pvc pipe with insulation, also to your benefit. Hope this helps, I know antifreeze isnt cheap, its also hard on pumps, but nothing sucks worse than frozen pipes. If it were mine and I had a concern. I'd try and hold out until next year and dig down a foot or two and add some insulation on top of the area where your pipes run under. A two foot wide piece of 2" pink styrofoam would put my mind at ease.
