i'm not sure i really understand either, but I think your saying that you want to put a bag inside the bucket so you can just pull out the bag and not have to dump it? Aren't the roots gonna be hanging into the bag making it hard to remove it? why dont you just drill a hole in the bucket and make a drain. kinda like a waterfarm that way you can see how much water is in the bucket too.

GENERAL HYDROPONICS

Check out the blue tube. try making the drain like that. Then you wont need a bag.- The House