Will a stock, say a McM, inletted for the Sunny Hill drop-box mag, still permit the use of the factory drop floor-plate, albeit w/o the additional capacity?

No, in a word. However, the factory can still be installed but you will need a shim under the factory floor metal because the SUnny Hill has a thicker front under the guard screw.

So, other than shimming the OEM floor plate, it would work?

I think you may also have to shorten up the mag box