I've done this a couple of ways. The first I was quite successful at avoiding much of any down escaping. I had a vacuum cleaner with mesh at one end of the tube so that some down could be sucked into the tube. Had a 3 oz size bag of down on a scale accurate to 0.5 grams. I could get maybe 3 grams up into the tube before the vacuum wasn't strong enough. I'd then move that 3 grams inside the tube, insert the open end into a baffle, close off the fabric around the tube, and blow it in.
I put 12 or so ounces into a quilt that way. That's over 100 loads transfered. I am not revealing any trade secrets when I tell you that Adam and Jenny ARE NOT stuffing the HammockGear quilts this way!
In recent projects I stuff more into a transfer tube out of that same bag, by hand. I can push up the amount moved in a load to a lot more. The fastest way is to get my hand down in the bag and stuff, but then some of the down floats out. Now the vacuum cleaner is used to chase down the strays. With a little attention, the down that does come out is due only to static electricity causing it to cling to the hair on my hands and wrists.