Did you think of getting a Kifaru E&E it's specifically designed to be piggybacked. It's a nice little back on it's own.
Do you have one? Do you like it? How do you carry it, when, and what fits into it?
I have not seriously considered the E&E for several reasons. I was on a trip with someone who had an E&E fully decked out with accessories, and as a day bag, as he had it, it was absurdly heavy. It and all that was hung off it were 1000D Cordura, often in double layers, and all covered in PALS. Even with all the stuff hung off the E&E, the PLCE pouches had more volume and were lighter, even in their own 1000D material.
I do have a Tailgunner I which I use for work, and have used as a day bag in the UK in summer, and I like it, but it really lacks the volume for rain gear + lunch + water + tools/camera/binos. Did look at an E&E over the top of the Tailgunner, but was not happy with the Tailgunner as a lid for my Zulu, nor with having to carry it in a Pod strapped to the bottom of the pack when not in use. All too many extra fuss for too little function, I thought.