At the barn that I work at they are willing to do that. I'm sure that many of them would be willing to as long as it doesn't inconvenience any of the other workers that actually get paid for their work.
Sometimes they even let you work their, get paid, and ride their horses. That's how it is in my case. I do my work in the morning and get it done and then if there are any horses that need to be ridden, I can get on and usually the trainer/instructor is there to give me pointers, if not a lesson.
Not all barns offer that, but some do. Just look around. I agree with SonnyWimps in that you need to be upfront and tell them what you are looking for.