These days, with advance crews setting up outdoor jumbotron, I would expect some sort of portapotty facility outdoors.
I have no idea about chair leave-behind. If I was doing that, I'd leave it, and then exit same door and look for it. If it's still there, take it home. If it's gone, well, great! You won't get pinched for littering.
Wacky thing, I went to the venue’s website, but no info about the rally there. There is a bit if info in the FAQs about what’s allowed on site generally. It can vary by venue. Best course of action might be to call the venue and find out about specific do’s and don’ts. Ex: No SLRs allowed at the rally we attended, only fixed lens cameras, cellphone cameras.
Thanks.