You're a senior and soon you'll have the advantage of never seeing them again. Do what I'd do, stash a cheap horn and a mute in your luggage. Once the bus/plane gets rolling, there's not a lot anyone can say. Be discreet. Practice a little everyday. Unfortunatley, here's the rub. It just sucks that some baffoon is telling you you can't when you already know what you're going to do. Be respectful! If you do get approached and they say "You were told to not bring your horn!!" just say "Don't worry, this isn't the good horn that I perform with. This is a model to keep my lips in shape. if it gets hurt, at least it's not my good horn". If push comes to shove, let them have the horn (make sure you get a reciept from them that's signed by the person that the horn is in perfect working order and that way if it's damaged, they will be responsible) but you keep the mouthpiece to buzz on.
If they ask for the mouthpiece, simply tell them no.
You(or someone) asked if they can take their instrument and were told no.
There's an underlying lesson here that's far more important than the two performances you are to do.
Its a life lesson that many don't learn till much later in life. Hopefully you will learn from it today.
What's the lesson?
"Don't ask questions that have answers you may not want to hear".