I know you don't quite like her opinion, but Speed_Racer has a very valid point and she is not the only one who has it. It sounds like you already have decided that they will let you ride their horses, so I'm not sure why you'd ask.
Again, prepare in advance. Advertise yourself for catch riding. Personally, I wouldn't care who your trainer is, the trainer is not the one riding my horse. Advertise and show your or your trainers horses. Exercise horses for people, etc. Don't just show up and expect there to be horses for you.
Your last post you say you're already doing it though...so I'm curious why you'd ask? Maybe I missed the point..