I'm not sure where you are located or your boarders, but if my barn did that I would be mad. We pay oodles of money every month to keep our horses there and use the facilities. If I want to pay a different coach to come teach me, what difference is it really making if I used one of the ones you have or a different ones. I'm the one using the facility. I kind of feel like it's the same way if I use a different farrier, vet, ect.... I'm the one paying the board, it's my horse, and my money I pay every month to keep him there.
If you want to charge a fee for out of house coaches, it would have to be included in the overall board prices not per session. But do you not consider use of facilities as included in your boarding fees?
And I don't think jumping is increasing anyone's risk, especially if they have a coach there, I think that would lower the risk.
I didn't mean to sound mean, but if my BO tried to charge me for bringing in a coach, I would laugh at him and say ok, I'll just go get a stall across the street.
It's always something interesting....