Thinking outside the box is good, but unfortunately your idea absolutely will not work. You create a centrifugal force by spinning something. While it's spinning, inertia tries to make the object continue on the path it was taking. Because it's spinning, this force takes the form of a tangent ray in the direction of the motion. Because every particle is moving at the same time, there are an infinite number of these tangent rays if you were to draw the force on paper, and they work together to produce an overall outward force.
Anyway, physics lesson aside, the key is that this force is only present when you move the object. No motion, no force. By turning the plants a quarter turn a week you would be providing a minuscule amount of force for an even shorter period of time. It would be like trying to get your plant to bend over by thumping the pot on the side every day.
With that out of the way, I think the OP is very clever, but I have the same concerns about speed. While it's small, 45 RPM is fine, but when it gets bigger, those forces are going to get amplified through the larger plant and it is possible that it could be damaged. Modifying it to spin slower will also cause stress on the motor, especially with such a huge weight on it (it was designed with vinyls in mind, not a bucket of wet dirt). By increasing resistance you're not just reducing the speed of the motor, you're reducing the overall output as well, which means the torque will be reduced by the same percentage. My guess is that the turntable would burn out in a matter of days if not hours or even minutes.
You could very likely build something like this pretty cheap, though. All you need is an electric motor you can tune down to 1-10 RPM, with sufficient torque to spin a wet pot without burning out. You could use simple little caster wheels mounted on a rigid wooden frame to support the weight of the plant and further reduce stress on the motor. Ball bearings are obviously a better choice, but that's a lot more work and more expensive too.