A lot of 135 tour ops, its just up to the captain anyways. This summer when I was flying up in AK, I showed up a tour operator near Denali and asked about riding along on a flight if there was a seat available, since I was flying then for a part 135 operator.
They said sure, just be there at 8am in the morning, and I got to go in a turbine Beaver that landed on a glacier next to Denali where the climbers basecamp was.