T-bone, I cannot speak from the experience of trying your idea but I foresee many downfalls -
* Consider the variations in cutouts required for SP-10, 301, 124, etc. Your idea would require the largest cutout of these for your donut, something that might not allow as much support or mass as a custom cut out for one of the smaller footprints.
* Different mechanisms generate different resonances and vibrations, thus requiring different materials. For example, the famous Lenco site began recommending layered MDF, although later a sandwich of ply and MDF has been favored. At least one SP-10 site recommends against MDF. So, what materials do you use?
* Considering the potential mass of your plinth and some of the motor units (plus the top board), switching back and forth like armboards is hardly comparable.
* From all my reading on plinth design and building, one point is consistent - the need to securely couple the motor unit, plinth, and arm. Some folks use both screws and glue in constructing plinths. Designing for a removable top board might require 20 or more screws to truly secure it to the base (donut) plinth. How interchangeable would that be?
So to close with your own words, my thought is this would be a stupid idea.