First of all, I don't think there's a way to do this without using multiple materials.
Secondly, unless you are using all lambert shading models, you'd also need to key the diffuse and/or specular attributes. Glass or plastic, for example, are transparent but not invisible.
You can set up a transparency control panel pretty easily.
Create the shaders you want, and call them something like shader1, shader2, etc.
Create a locator, and add several attributes to it; one for each shader whose transparency you want to key. Name each attribute something like "shader1Trans" "shader2Trans" etc., make the attribute a float data type and set the min max to 0 and 1.
In the connection editor, connect the new locator attributes one at a time to the transparency R, transparency G and transparency B of the corresponding shader. Now, when you select the locator, you have an array of attributes, each keyable from 0 to 1, that control the transparency of your shaders. You can hook these up to the specular or diffuse attributes as well.
If you want, you can use this mini-script to create a custom window for your new controls:
if (window -exists transControl) deleteUI transControl;
window transControl;
columnLayout;
attrFieldSliderGrp -min 0 -max 1 -at locator1.shader1Trans;
attrFieldSliderGrp -min 0 -max 1 -at locator1.shader2Trans;
showWindow transControl;
Copy and paste that script to your shelf and that'll give you a nice window with sliders that control everything you want. (the single quote at the beginning of the script is the key above the TAB key - not the regular quote ket)
For the sake of neatness, you can select the locator and open the Channel Control window, then remove all the attributes except the ones you made; then you can put the locator on a layer and make it invisible. You'd never need to select it - your custom window now does everything you need it to. Incidentally, if you right-click on any of the slider labels, you get a little menu that enables you to set keyframes right from there.
I'd only use this method if you need to keep keying the transparencies in the same scene over and over again ... the old "bang/buck > 1" method of rigging.
Good luck
David
