I think the weight is really the biggest problem. Even fairly light DSLRs weigh a bit, and you'll soon start stacking counterweights. (Which has a lot of potential to amplify vibrations :)
Most mounts can take the amount of weight of your scope plus a DSLR as you probalby have more mount than scope (if you've purchased well), and if you're using an equitorial mount, you using counter weights already. The worst part of the weight is on the focus tube itself. Whenever I'm using my DSLR, I determine where the scope will be pointing and position my scope and camera rig so that it is never going to rotate so that the camera is on top facing down putting all of it's weight on the focus tube screw locks. I always try to position it so that it is hanging to the side. I've made that mistake only once.
I still don't get why "camera on top" is problematic, though - unless you're using Newtonian/Refracting? Is there something I'm missing where I'm setting myself up for disaster due to my ignorance?
Oh, I forget people still try to use AltAz for imaging. You have my sympathies for the troubles endured.
I didn't have the focus tube screws locked tight enough. I walked away from my rig for the comforts of my bed while the gear worked through the night. When I returned to the rig, I found that due to the way the rig was rotated in/on the mount ended with the camera directly ontop of the optical tube. Since the screws were not tight enough, the weight of the camera pushed the focus tubes into their most compact state. This clearly blew focus on all of the images from when ever it started to move. I have some lovely out of focus shots of a total eclipse for sale if you're interested ;-)
After that, I learned to manualy rotate my rig through the motion it will undertake through out the night in advance so that the camera is not side slung with the weight never to come straight down like that. I also try to arrange it so that it is always adding tension to the weight so that it is not pulling against the motion.
