Right but it illustrates the point that just because something is allowed on public transport doesn't mean its allowed on Disney transport. The poster I quoted equated Disney transport to public transport you would find in a city. Disney transport is designed to make it easy to travel around disney property (and maximize the time and money you spend there) whereas public city transport is designed to move you between cities, around the area to multiple facilities.Your luggage argument is irrelevant. The OP is not talking about luggage. And I don't know why shopping bags from a non Disney store should be any different from shopping bags from a Disney store. Or bulky backpacks, diaper bags, or strollers for that matter. People go to the Hess station for a beer run all the time, and use Disney transport back to their resort.
Having said that, I think its penny wise and pound foolish to waste the time and energy to do what the OP proposes.