I put in a mile with 70lbs total (chair plus 60lbs). The plastic frame is designed to carry more than any of us intend to whitetail hunting. I lashed 60lbs of dumbells to it and it handled it fine. Load lifters wouldn't be impossible to implement, but I haven't quite figured out how to do it. It balances pretty well otherwise. Here's what you have to take into consideration: You're not packing 60lbs every time - just when you kill a deer. Given the choice of packing one with the hybrid as is, or dragging, sledding, or carting a deer out of rough terrain, hinge cuts, CRP, all that jazz, I'd choose this by a mile. It's not even close.
There are a ton of different options on how to lash the load to the frame and or the webbing. I used 15' or so of 1" webbing weaved all through the weights and the frame to secure it. For deer - I have a pretty unique little bag I pack meat out in - maybe one day I'll show you guys it's other uses. But I am going to make up some lashing straps to secure it to the frame. I think the best way is to run two from where the waist belt goes through back of frame, under my bag(laying in the "shelf" that the straps from back to seat create) up around the bag to the top of the frame. Cinch those down, and then one or two horizontal lashing straps. the setup could be made for less than 10.00 and would require no sewing. I hope to have mine done before November trip and I'll share. Worst case grab a 10' piece of paracord and draw it all down with that.
I will do my best to pack a deer out on this trip haha. Mine or someone else's, and I'll get some film of loading it up, and carrying it.