Just an idea, but what about calculating the distance from your camera look at point to your camera base point. Then just check the distance and see if it is still within
bounds. This involves a bit of math, but its worth it in my opinion.
I do this for one of the projects I'm working on. It gives great results with zooming. With a little extra work you can also use this for 3D rotations.
This probably isn't what your looking for, but it maybe a good idea to learn how the camera moves in 3D space.
I know it has helped me a lot.