Be aware that when they say "black tie", they may really mean that everyone will wear a tux and that's it. Some may be more lenient, but many may be afraid of the slippery slope - if we allow highland attire this time, what will someone want the next time, and before you know it the event no longer resembles what they want.

Yeah, that may seem overly restrictive, but it's their party. They get to set the rules, whether you like them or not.

That being said, you're following the right course by asking the hosts about your desires.