Andrew, the foot box opens up 24" from the bottom of the bag. We have tried to strike a balance between versatility and efficiency (we don't want people to have cold feet using our bags). The bag can be used laid out flat in warmer weather. This works best in a bivy, the bivy will keep the lower portion of the bag located on top of you while you sleep.
Frank, we are not doing any custom work right now. One thing about down bags is the ability to move the down around in the baffles, you could buy the warmer bag, and shift some down to the bottom for warmer weather (of course this would be heaver though). We will be introducing the bags in the 6' length first, followed by 6'6", then 5'6".