I'm not completely sure but I don't think there should be any problems. I've seen a lot of Spruce Meadow and high class Show Jumpers using hackamores on their horses. I don't know how long the shanks on their hackamores are but I doubt yours will be a problem.
Yea I've seen them riding in them too, I just wanna make sure before I start riding alot more and training cause I wasted 3 years with a bit on him and my trainer said it wasnt the bit it was him.. ect then I changed him to a hackamore and he's never been better! So I don't wanna keep riding him in the hackamore and find out I can't use it :/