You have to start using the words "risk" and "maintenance cost" when talking to managers then.
The main drawback to the solution you describe is not the "two days to develop", but doubling or tripling the complexity of that part of the application. It will be a pain to support and enhance in the future.
Manager: "Is it possible?"
You: "Yes it can be done, but you're basically making this application twice a complicated for this one feature. This makes deployment and support more complex, and if anything ever goes wrong, it will take a lot longer to even find the problem. If we ever want to upgrade the underlying software, this could break."
The main drawback to the solution you describe is not the "two days to develop", but doubling or tripling the complexity of that part of the application. It will be a pain to support and enhance in the future.
Manager: "Is it possible?" You: "Yes it can be done, but you're basically making this application twice a complicated for this one feature. This makes deployment and support more complex, and if anything ever goes wrong, it will take a lot longer to even find the problem. If we ever want to upgrade the underlying software, this could break."