Disclaimer: the following are very preliminary designs and they're not suppose to be free of problems, and instead, it should have problem, so don't follow them unless you're 100% clear about what you're doing. The author will not be responsible for any problem directly or indirectly caused by the information here.
DC Geared motor can be reversed by changing the polarity of the inputs, and its speed can be controlled via varying input voltage or PWM. To changing the polarity of the inputs, a simple design can be used like the following:-
It consists of two buttons, each has three inputs, when the button is pressed, the top input is connected to either one of the other inputs, and if the button is released, it will be connected to another inputs.
A (variable) resistor can be connected in the position of the red ellipse, to control the voltage applied.
Suppose the left button is pressed and the right button is released, the green lines are connected and the red lines are disconnected. Therefore, output A will be -ve and output B will be +ve.
Suppose the left button is released and the right button is press, the red lines are connected and the green lines are disconnected. So, in this case, output A will be +ve and output B will be -ve.
So, you see, by pressing different buttons, the polarity is reversed. For example, a motor is connected to it, and the shaft is connected to the DEC knob of the mount, pressing one button will turn it CW and the other one will turn it in CCW. Therefore, to duplicate the about circuit in RA, we will then have control over RA adjustment as well.
This simple setup will allow motorized manual guiding, but it's hard to incorporate into a autoguider port.