Does anyone know if they allow you to hold a DSLR during the rides? I'm not intending on taking pictures. If not, do they have a secure location to keep it?
Yes, you certainly can. They're kind of the opposite of Universal, which is incredibly strict about bringing things on; you can bring anything on a Disney ride. I even shoved my (folded-up) tripod next to me on Space Mountain once.
If you want to, you can take pictures, too, but make sure the camera is secured tightly! I did one time have someone come out at the beginning of Rock 'n' Rollercoaster because I think they thought someone was doing something like holding the camera up to their eye as it was about to take off - obviously don't do that, you're best aiming blindly! I have taken shots while waiting for the fast take-off several other times and nothing happened those times.