I decided to uppot a 1 gal cutting today because it clearly overgrew the container. It was one of the fastest growers from that batch. 
To fly back in time, here is a short story...

When I was moving the cuttings from the cups to 1 gal pots, I used wood or rocks to block the huge drainage holes so the soil wouldn't escape. At the end of the up potting I ran out of the material and decided to use the packing pink soft popcorn that was in a box nearby. The particle size was perfect so why not? I put about 2 layers of that popcorn, filled the rest with the soil and forgot about it... until today...

I was very surprised to see a lot of healthy roots circling the popcorn. They clearly liked the extra air. 
Is it a lesson for the future?... Anybody has done this before and noticed a better result?

How much butter do you add?

Oh, well, just the right amount, precisely calculated, to keep the babies happy ...

GreenFig, yes! I did the same thing when up-potting to my larger containers last month. I wanted to slightly block the holes in the bottom as well and used a few handfulls in the bottom of each pot,,then the potting mix, then the young tree. Have had very good success with this procedure and it does provide a nice drainage layer. When moving to inground or a larger pot, i just leave the popcorn in place.