So, is the foliage feeding tree or are the roots feeding the tree (foliage). If I knew that the roots were contributing more then I would trim some foliage. Maybe? I'd like t feel that I'm doing as much as I can, it's a special tree.
The roots provide water, nutrients and hormones.
The foliage provides food (carbs via photsynthesis) and hormones.
The problem with a newly collected tree is that the roots that were collected may be inadequate or compromised so that they can't provide enough water to the foliage to keep up with the rate of transpiration. Some people suggest reducing foliage, but if you aren't careful you weaken the tree so that it doesn't have enough energy to recover. In a perfect world you want to keep as much foliage as possible... but you can't let it dry out!
Best practice is similar to what you have been doing - plant the tree up in a loose, free-draining soil mix that is at least 80% inorganic - pumice, decomposed granite, crushed lava, etc. Then place the tree outside in a sheltered spot where it gets strong indirect light but almost no sunlight - as bright as possible with no sun. Place a humidity tent over the tree made out of translucent plastic. Your garbage bag idea is directionally correct but you don't want the plastic to lay on top of the foliage if you can avoid it. See if you can build a wire frame or something to hold the plastic away from the tree. You want the opening of the bag to face downwards and you want to shroud the tree like it is in a tent. Once you get this set up, try to mist the foliage frequently to reduce the transpiration rate of the tree, and consequently reduce the demand on the roots. You want the inside of the tent to be a mini-greenhouse and very humid. Be careful during this time to not overwater the soil - because the roots are compromised and reduced they will not be taking up very much water - if any - and it is easy to overwater a tree that has just been collected. Having an open non-organic mix really helps at this point with drainage.
If it stays green and supple for a month, you may start to feel optimistic, but don't remove the humidity tent until you see it starting to push new growth - which is the sign that the roots are growing. This could easily be three months. Don't move it into full sun in one go, either. Take baby steps.