If a store offers a publicly accessible rubbish bin, then it would be ethical to ask the store for permission to discretely transfer their product to your own containers inside or near the store and discard the packaging in their bin. In some cases it's already normal to discard packaging near or in the store, such as in a food court or mall.
Your suggested approach is an interesting one, but is unlikely to be seen as particularly constructive or positive. Stores certainly wouldn't welcome additional mess to clean up (such as spilled product), and may want to retain a more professional and polished image.
A Different Approach
You are clearly willing to put substantial effort into the goal of reducing waste, which is fantastic. A more constructive approach would be to open communication with stores, as @martin-maat suggests. If you researched some options for improvement that could work for the store, they might be willing to work with you.
For example you could ask what it would take to get them to change to recyclable packaging, or to allow bringing of one's own container. They might agree to do it if you get support from 100 or 1000 customers.
In the case of a high complexity store like a supermarket, there's not going to be a quick fix, especially when changing their processes involves increases in cost or inconvenience.
The Bigger Picture
There are waste management groups that look at this kind of problem from a high level perspective; perhaps you should try to find one and see what you'd need to do to get involved?