Thats an old wives tale that has been debunked for years. Some snakes get cage defensive but that has nothing to do with feeding. A snake will perk up when you open the cage but once there is no food and it doesnt smell food, it will mellow out. I have fed all my snakes in their cages for years and never had a problem except when i did something stupid like forget to wash my hands after handling FT. Even my retic is fed in his cage and i have never had an issue.
My question to the guys who say you need to feed outside the cage, want come and try that with my retic or boas? I would love to watch you try and move them after they smell food especially my retic who has slammed his cage doors a couple times going for food.
And to the OP, i wouldnt worry too much as long as your husbandry is correct. My girl has gone off food for 4-6 months before with no problems. She actually pissed and shed in that time too. And eating some aspen wont hurt them. All of my snakes have eaten aspen before. Some people will say it can cause impaction. This is true if your husbandry is wrong. A well hydrated snake will easily pass a few pieces od aspen. Their stomach acids break down bone...i think it can break down a few slivers of wood lol. After all, imagine what they eat in the wild.
If you are worried, just feed them on a sheet of cardboard or newspaper. I would avoid paper towels as that can wrapped in the prey.