Any plumbing problem can be solved with the appropriate amount of cubic dollars. And that is the problem. You are asking for such a deviation from their standard construction that they just don't want to deal with it. For them, it would involve a big change order with their plumbing contractor, sourcing the valve from someplace they may not have an account ( hard to imagine for such a big builder, but $$$ is everything), and deviation from their contstruction schedule.
Opening up tile is POSSIBLE but not a good idea, as it will never look "original". Cutting into the waterproof deck of the tub is very problematical. Is there a stand up shower in your master in addition to the roman? Maybe you just rethink your needs.