Here's my thought, see if it would work in this case - add a Virtual Beauty pass in the Corona Multi-pass dialog, and keep Denoise Amount for that at 0 (or any desired lower value than you will use in the main Beauty pass). Then when you save the render you will have your regular Beauty pass which is as Denoised as you want, and can use a mask to blend that with the Virtual Beauty pass with no denoising. That's the approach shown in the 3ds Max tutorial videos, and would work the same here for C4D.
You'd need to be using the Daily Builds to have the Virtual Beauty pass, if I remember rightly.