While it is true that your hips can take the load of a heavy backpack, having the shoulder straps there to "anchor" the pack helps. In contrast, wearing a heavy hip pack (aka lumbar pack or fanny pack) can be uncomfortable.
For me, when the total load of my frameless silnylon pack is around 20 pounds or so, I hardly even feel the pack at all! While one should take care to pack properly, I don't think there's any concern about splitting up some of that weight and putting it into a lumbar pack. And reading your posts, you are obviously aiming for a very light weight pack load as well.
Also, with a UL silnylon pack, there's really no need for a separate day pack or hip pack at all. For day hikes, the main pack is more than light enough.