I think, because of its minimalist construction, the only difference between my Buttermilk and an UL pack is some durability in the bottom section and the luxurious back foam panel. My Buttermilk is a hair under 2lbs without the frame sheet, so the 1lb tradeoff for the durability and the comfort seems like weight well spent. That is relevant, since a lot of frameless UL packs will feel very different than mine, and the associated issues will be different. Perhaps.
When I load it up with 13lbs as my baseweight and then add water, I can feel the weight on my shoulders but also on my hips, and the feel of the backpanel is no different from when I'm using the frame. One thing I forgot to consider, however, is the 5lbs of camera gear I have on my shoulders, resting in front of me on a chestpack. My backpack might weigh sub-20, but my shoulders are carrying that +5lbs.
Still, if the foam sheet redistributes to my hips, this seems possibly doable.
I think testing is my only surefire way to solve this, but I wondered if anyone had a similar experience worth sharing. I'm loving the feedback: THANK YOU!