1. Dont just call them up and ask over the phone. If you need to call them ask to make an appointment with one of the managers about using the facilities for a rare sport. Avoid giving them details over the phone.
2. Take along your stilts to meet the manager, and also a video of some other gym sessions that have happened elsewhere. The more the better as this creates the question: Why cant I do this here?
3. If they ask about liability insurance, run for the hills. Unless you have some.
4. Having prepared your own risk assessment will show you're taking it seriously and not just going "please can I use your hall".
5. If it gets rough and its not going well, try to turn it on its head and ask who else you could speak to or even put them on the spot and ask why they wont support something like this. Direct questions are hard to answer.
6. If they want a demonstration, DO NOT FALL OVER. As blatently obvious as this is, dont ever underestimate it. You fall, they will say no forever. Infact I'd advise to not do any tricks at all or jump high.
Also if the demonstration happens in the hall, insist on putting gym mats down or the BANG BANG BANG noise will put them right off.