I have never had any mixture or plain blended fruit that was to runny to make fruit leather, and I've never had a failed batch of any fruit. The last batch of blueberries/wild blackberry mix I did this summer I had a huge jug full of blended berries and by the time I was on the last few sheets I could hardly pour it out because it was already the consinstency of jam.
Most fruit has at least some pectin. Theoretically you could make some tasty fruit leather from grape juice if you could get it to stay put, which is probably where the applesauce theory comes from. It's more for making it less runny for pouring sheets than
consistancy of the finished leather. I've also heard of adding yogurt but I've never done it. I guess my advice would be to play it by ear, blend some stuff up and if it's too runny then add the applesauce. My
experience with raw apples is that they quickly turn to
apple crisp if you aren't careful, but its also tasty -
great addition to salads.