You don't have to worry about your snake ingesting substrate anyway - so that's a moot point.
As for them learning to associate the opening of their enclosure with feeding - maybe if that's the only time you interacted with them....and I stress maybe.
But in 30 years of keeping snakes I've never had an issue with snakes associating opening the tub/door with food. There are species I keep in pairs and must feed separately - now those snakes have bit me when I go to move them after their meals, but never upon opening the door.
I'm just in awe of all you people who have these amazingly intelligent snakes that do things mine don't - like have issues ingesting substrate, become blood thirsty biting machines when the door is opened, display cognitive learning behaviors, etc.
I have a serious question for all you separate tubbers:
How is it that your snakes are smart enough to associate the opening of the door with food - especially when you also open the door to clean, change water, cuddle and stroke them, etc. but magically don't correlate the feeding tub with food? Especially since that's all the feeding tub is used for - feeding.
How you feed your snakes is up to you, but you all seem to be going to an unnecessary hassle in an effort to head off a non-existant problem.
At some point you may move beyond ball pythons and might find out the hard way that hungry snakes should not be moved before and after their meals.