I've done a lot of backpacking/hiking in minshoes with packs up to 40lbs. I haven't noticed any negative difference in my feet with the added weight. In fact, since switching to minimal my feet, legs, hips and back feel better than they ever have on long backpacking trips. My longest and most technical hike/backpack in minshoes was the Inca Trail in Peru where I wore my Treksports the whole time (4 days, thousands of feet of elevation gain/loss and a total about about 26 miles).
I was nervous about a long backpacking trip in minshoes at first too, and I did pack in my heavy hiking boots for my first longer trip just in case, but I was pleasantly surprised that it was completely unnecessary. I hike with a friend that also converted to minimal (hiking/walking but not running) last year and her experience was the same. I know a sample of two doesn't mean it's going to be the same for everyone so if you're worried specifically about carrying more weight while still wearing the Unshoes, it might help you decide how your feet will handle it if you pack a heavier pack and go for a long day trip carrying the weight you would otherwise take on an over night trip and see how your feet feel rather than test it out on your big trip?
Most of my hikes are in the Vibram Treksports, but I've done a few in Merrel trail gloves as well. I'm planning on trying out hiking in Unshoes this summer as soon as they are delivered.