We've been working through different designs for this as well. It has proven useful for us to carry this "modularity" into the field of caches as well. Our primary concern was quick resupply for the consumables, as well as pre-positioned stocks of everything (e.g. you must leave with the clothes on your back).
Consequently our BOB's are fairly closely styled to yours, GreekMan. To supplement them and take the "modular" concept further, should we need resupply it is located at one of our cache points. There are also independent "islands" that are complete 30 day stand-alone caches with food, water, shelter, clothing, firearms, ammunition, medical supplies, and more for a family of 6.
When I lived in the Denver area for college we had 3 blizzards come in one winter (2006). Part of our emergency kit was bottled water. Our low tech strategy for keeping them from becoming ice blocks was not geared toward prevention. It would have been virtually impossible for us to fight that constant battle. Bringing them in the house every night without forgetting vs. the one time you actually did get stuck somewhere seemed pretty likely that we'd still end up with ice blocks, or no water.
So we took a large cooler, lined it with foam mattress material. Inside that we lined it with solar blanket. Then we had enough room to put 8 bottles of water (each 75-85% full) and 2-4 chemical hand or foot warmers. This is in addition to having solar blankets, duct tape, a small camp stove, and full size blankets. Our intention was that in an emergency that required us to have drinking water (instead of ice), we'd activate the handwarmers, throw them in the cooler, close the lid and wait 10-20 minutes.
While we were waiting for the water to thaw, we'd build a small tent inside our vehicle with the solar blankets and tape, use the campstove (well ventilated) to warm up the enclosure, and wrap up inside the blankets. We never had to use the enclosure idea, but the water bottles rarely froze in the cooler, and if they did the warmers would have melted some of the water for drinking.
I like the direction you are all thinking, and look forward to seeing your progression in this preparedness.