I have a few thoughts about this:

• I wouldn't recommend anyone do substantial backpacking of any type in a minimal shoe until they had experience with minimal shoes in everyday life, and done a lot of day hiking in them. Supporting the extra weight of the backpack for long periods of time requires developed muscles in the legs, hips, and feet.
• I would still recommend a low-top shoe, unless the top is flexible and offers a good range of motion. If you restrict the ankle too much, you end up putting stress in the knee, in directions it wasn't designed to carry stress. You will increase your risk of knee injuries if your ankle is locked.
• Assuming you are comfortable with minimal shoes, with 50 lbs on your back the part of your feet that will suffer the most will be the soles. You will want to make sure you have adequate protection for the trail conditions you will be walking on. If you are going to be spending a lot of time on jagged/sharp rocks, you will want to have enough under foot protection. If you will be walking on flat, soft trails, that won't be as much of an issue. Either way, I still recommend as close to zero-drop as you can get, which is often difficult to find in shoes with more under-foot protection. Altra shoes are quite good in this regard, though I have had some durability issues with them in the past.

Thanks for the quick and thoughtful reply, Damien! I have a year or so of everyday and gym-minimal shoe-wear, but zero miles of experience in minimal footwear while walking under load. Again, I appreciate your reply and will heed your advice!