I noticed that Giles and Anna, who are members of our forum who are trekking across South America as we speak, have one pack saddle that has self-adjusting bars. They don't seem to be having any trouble with it, but they don't appear to be packing it as heavily as some pack animals I've seen (and packed). They don't appear to be using the Halfbreed with it either. They are using it more like a crossbuck pack saddle, which is ok, I guess. Don't know why it wouldn't be.
They also have another style pack saddle that I've never seen before, pictured below. I say I've never seen one before, but I missed one just like it that sold on ebay
several weeks ago for about $25, because the owner didn't know what it was...I didn't either. Wasn't sure it was for a horse or mule, but thought maybe it was for a llama or something. Should have bought it.
You can find Giles and Anna's facebook page at "The Great Horse Trip".